diff --git a/client/src/components/inventory.tsx b/client/src/components/inventory.tsx
index cff2c1b..c89e043 100644
--- a/client/src/components/inventory.tsx
+++ b/client/src/components/inventory.tsx
@@ -6,8 +6,9 @@ import { faLock, faLockOpen, faBook, faHammer, faBan } from '@fortawesome/free-s
 import { GameIdContext } from '../app';
 import Markdown from './markdown';
 import { useLoadDocQuery, InventoryTile, LevelInfo, InventoryOverview } from '../state/api';
-import { selectInventory } from '../state/progress';
+import { selectDifficulty, selectInventory } from '../state/progress';
 import { store } from '../state/store';
+import { useSelector } from 'react-redux';
 
 export function Inventory({levelInfo, openDoc, enableAll=false} :
   {
@@ -50,6 +51,8 @@ function InventoryList({items, docType, openDoc, defaultTab=null, level=undefine
 
   const gameId = React.useContext(GameIdContext)
 
+  const difficulty = useSelector(selectDifficulty(gameId))
+
   const categorySet = new Set<string>()
   for (let item of items) {
     categorySet.add(item.category)
@@ -87,7 +90,7 @@ function InventoryList({items, docType, openDoc, defaultTab=null, level=undefine
         ).filter(item => ((tab ?? categories[0]) == item.category)).map((item, i) => {
             return <InventoryItem key={`${item.category}-${item.name}`}
               showDoc={() => {openDoc(item.name, docType)}}
-              name={item.name} displayName={item.displayName} locked={item.locked}
+              name={item.name} displayName={item.displayName} locked={difficulty > 0 ? item.locked : false}
               disabled={item.disabled} newly={item.new} enableAll={enableAll}/>
         })
       }
diff --git a/client/src/components/level.tsx b/client/src/components/level.tsx
index 117b0ab..9332eea 100644
--- a/client/src/components/level.tsx
+++ b/client/src/components/level.tsx
@@ -20,7 +20,7 @@ import { InfoProvider } from 'lean4web/client/src/editor/infoview';
 import 'lean4web/client/src/editor/infoview.css'
 import * as monaco from 'monaco-editor/esm/vs/editor/editor.api.js'
 import './level.css'
-import { useStore } from 'react-redux';
+import { useSelector, useStore } from 'react-redux';
 import { EditorContext, ConfigContext, ProgressContext, VersionContext } from '../../../node_modules/lean4-infoview/src/infoview/contexts';
 import { EditorConnection, EditorEvents } from '../../../node_modules/lean4-infoview/src/infoview/editorConnection';
 import { EventEmitter } from '../../../node_modules/lean4-infoview/src/infoview/event';
@@ -36,7 +36,7 @@ import { Button } from './button'
 import Markdown from './markdown';
 import {Inventory, Documentation} from './inventory';
 import { useGetGameInfoQuery, useLoadLevelQuery } from '../state/api';
-import { changedSelection, codeEdited, selectCode, selectSelections, progressSlice, selectCompleted, helpEdited, selectHelp } from '../state/progress';
+import { changedSelection, codeEdited, selectCode, selectSelections, progressSlice, selectCompleted, helpEdited, selectHelp, selectDifficulty } from '../state/progress';
 import { DualEditor } from './infoview/main'
 import { DeletedHint, DeletedHints, Hints } from './hints';
 import { DeletedChatContext, InputModeContext, MonacoEditorContext, ProofContext, ProofStep, SelectionContext } from './infoview/context';
@@ -389,6 +389,9 @@ function LevelAppBar({isLoading, levelId, worldId, levelTitle, toggleImpressum})
   const gameId = React.useContext(GameIdContext)
   const gameInfo = useGetGameInfoQuery({game: gameId})
 
+  const difficulty = useSelector(selectDifficulty(gameId))
+  const completed = useAppSelector(selectCompleted(gameId, worldId, levelId))
+
   const { commandLineMode, setCommandLineMode } = React.useContext(InputModeContext)
 
   return <div className="app-bar" style={isLoading ? {display: "none"} : null} >
@@ -414,7 +417,8 @@ function LevelAppBar({isLoading, levelId, worldId, levelTitle, toggleImpressum})
           </Button>
         </>}
         {levelId < gameInfo.data?.worldSize[worldId] &&
-          <Button inverted="true" to={`/${gameId}/world/${worldId}/level/${levelId + 1}`} title="next level">
+          <Button inverted="true" to={`/${gameId}/world/${worldId}/level/${levelId + 1}`} title="next level"
+          disabled={difficulty >= 2 && !completed}>
             Next&nbsp;<FontAwesomeIcon icon={faArrowRight} />
           </Button>
         }
diff --git a/client/src/components/welcome.css b/client/src/components/welcome.css
index c57f6e9..802b03d 100644
--- a/client/src/components/welcome.css
+++ b/client/src/components/welcome.css
@@ -20,7 +20,9 @@
 }
 
 .welcome-text {
-  padding: 20px;
+  padding-left: 20px;
+  padding-right: 20px;
+  padding-bottom: 20px;
 }
 
 i {
@@ -91,6 +93,10 @@ svg .level:hover .level-title {
   opacity: 1;
 }
 
+svg .disabled {
+  cursor: default;
+}
+
 /******************/
 /* Privacy Button */
 /******************/
diff --git a/client/src/components/welcome.tsx b/client/src/components/welcome.tsx
index 35122bf..34265ea 100644
--- a/client/src/components/welcome.tsx
+++ b/client/src/components/welcome.tsx
@@ -4,17 +4,17 @@ import { Link } from 'react-router-dom';
 import { useNavigate } from 'react-router-dom';
 import { useSelector } from 'react-redux';
 import Split from 'react-split'
-import { Box, Typography, CircularProgress } from '@mui/material';
+import { Box, Typography, CircularProgress, Slider } from '@mui/material';
 import cytoscape, { LayoutOptions } from 'cytoscape'
 import klay from 'cytoscape-klay';
 import './welcome.css'
 import { FontAwesomeIcon } from '@fortawesome/react-fontawesome'
 import { faGlobe, faHome, faCircleInfo, faArrowRight, faArrowLeft, faShield, faRotateLeft } from '@fortawesome/free-solid-svg-icons'
 import { GameIdContext } from '../app';
-import { selectCompleted } from '../state/progress';
+import { selectCompleted, selectDifficulty } from '../state/progress';
 import { useGetGameInfoQuery, useLoadInventoryOverviewQuery } from '../state/api';
 import Markdown from './markdown';
-import WorldSelectionMenu from './world_selection_menu';
+import WorldSelectionMenu, { WelcomeMenu } from './world_selection_menu';
 import {PrivacyPolicy} from './privacy_policy';
 import { Button } from './button';
 import { Documentation, Inventory } from './inventory';
@@ -31,12 +31,17 @@ const padding = 2000  // padding of the graphic (on a different scale)
 function LevelIcon({ worldId, levelId, position, completed, available }) {
   const gameId = React.useContext(GameIdContext)
 
+  const difficulty = useSelector(selectDifficulty(gameId))
+
   const x = s * position.x + Math.sin(levelId * 2 * Math.PI / N) * (R + 1.2*r + 2.4*r*Math.floor((levelId - 1)/N))
   const y = s * position.y - Math.cos(levelId * 2 * Math.PI / N) * (R + 1.2*r + 2.4*r*Math.floor((levelId - 1)/N))
 
+  let levelDisabled = (difficulty >= 2 && !(available || completed))
+
   // TODO: relative positioning?
   return (
-    <Link to={`/${gameId}/world/${worldId}/level/${levelId}`} className="level">
+    <Link to={levelDisabled ? '' : `/${gameId}/world/${worldId}/level/${levelId}`}
+        className={`level${levelDisabled ? ' disabled' : ''}`}>
       <circle fill={completed ? "#139e13" : available? "#1976d2" : "#999"} cx={x} cy={y} r={r} />
       <foreignObject className="level-title-wrapper" x={x} y={y}
               width={1.42*r} height={1.42*r} transform={"translate("+ -.71*r +","+ -.71*r +")"}>
@@ -58,6 +63,8 @@ function Welcome() {
 
   const inventory = useLoadInventoryOverviewQuery({game: gameId})
 
+  const difficulty = useSelector(selectDifficulty(gameId))
+
   // When clicking on an inventory item, the inventory is overlayed by the item's doc.
   // If this state is set to a pair `(name, type)` then the according doc will be open.
   const [inventoryDoc, setInventoryDoc] = useState<{name: string, type: string}>(null)
@@ -137,9 +144,13 @@ function Welcome() {
         nextLevel = 0
       }
 
+      let worldDisabled = (difficulty >= 2 && !(worldUnlocked || worldCompleted))
+
       // Draw the worlds
       svgElements.push(
-        <Link key={`world${worldId}`} to={`/${gameId}/world/${worldId}/level/${nextLevel}`}>
+        <Link key={`world${worldId}`}
+            to={worldDisabled ? '' : `/${gameId}/world/${worldId}/level/${nextLevel}`}
+            className={worldDisabled ? 'disabled' : ''}>
           <circle className="world-circle" cx={s*position.x} cy={s*position.y} r={R}
               fill={worldCompleted ? "green" : worldUnlocked ? "#1976d2": "#999"}/>
           <foreignObject className="world-title-wrapper" x={s*position.x} y={s*position.y}
@@ -164,9 +175,7 @@ function Welcome() {
     <Split className="welcome" minSize={200} sizes={[40, 35, 25]}>
       <div className="column">
         <Typography variant="body1" component="div" className="welcome-text">
-          <Button inverted="false" title="back to games selection" to="/">
-            <FontAwesomeIcon icon={faArrowLeft} />&nbsp;<FontAwesomeIcon icon={faGlobe} />
-          </Button>
+          <WelcomeMenu />
           <Markdown>{gameInfo.data?.introduction}</Markdown>
         </Typography>
       </div>
diff --git a/client/src/components/world_selection_menu.css b/client/src/components/world_selection_menu.css
index 5a55220..48e6d54 100644
--- a/client/src/components/world_selection_menu.css
+++ b/client/src/components/world_selection_menu.css
@@ -9,3 +9,34 @@
   margin-right: .4em;
   margin-bottom: .2em;
 }
+
+
+.world-selection-menu .slider-wrap {
+  display: inline-block;
+  width: 100%;
+  /* min-width: 16em; */
+  padding-left: 3em;
+  padding-right: 3em;
+  margin-left: auto;
+  margin-right: auto;
+}
+
+
+/* Test for mobile `title`s */
+@media (pointer: coarse), (hover: none) {
+  [title] {
+    position: relative;
+    display: inline-flex;
+    justify-content: center;
+  }
+  [title]:focus::after {
+    content: attr(title);
+    position: absolute;
+    top: 90%;
+    color: #000;
+    background-color: #fff;
+    border: 1px solid;
+    width: fit-content;
+    padding: 3px;
+  }
+}
diff --git a/client/src/components/world_selection_menu.tsx b/client/src/components/world_selection_menu.tsx
index 39d4280..e75c695 100644
--- a/client/src/components/world_selection_menu.tsx
+++ b/client/src/components/world_selection_menu.tsx
@@ -4,14 +4,15 @@
 import * as React from 'react'
 import { useStore, useSelector } from 'react-redux';
 import { FontAwesomeIcon } from '@fortawesome/react-fontawesome'
-import { faDownload, faUpload, faEraser } from '@fortawesome/free-solid-svg-icons'
+import { faDownload, faUpload, faEraser, faGlobe, faHome, faArrowLeft } from '@fortawesome/free-solid-svg-icons'
 
 import './world_selection_menu.css'
 
 import { Button } from './button'
 import { GameIdContext } from '../app';
 import { useAppDispatch, useAppSelector } from '../hooks';
-import { deleteProgress, selectProgress, loadProgress, GameProgressState } from '../state/progress';
+import { deleteProgress, selectProgress, loadProgress, GameProgressState, selectDifficulty, changedDifficulty } from '../state/progress';
+import { Slider } from '@mui/material';
 
 /** Only to specify the types for `downloadFile` */
 interface downloadFileParam {
@@ -35,12 +36,35 @@ const downloadFile = ({ data, fileName, fileType } : downloadFileParam) => {
   a.remove()
 }
 
+// <div><Button inverted="false" title="back to games selection" to="/">
+// <FontAwesomeIcon icon={faArrowLeft} />&nbsp;<FontAwesomeIcon icon={faGlobe} />
+// </Button>
+// <Slider min={0} max={2}></Slider>
+
+/** The menu that is shown next to the world selection graph */
+export function WelcomeMenu() {
+
+  function label(x : number) {
+    return x == 0 ? 'Easy' : x == 1 ? 'Explorer' : 'Strict'
+  }
+
+
+  return <nav className="world-selection-menu">
+    <Button inverted="false" title="back to games selection" to="/">
+      <FontAwesomeIcon icon={faArrowLeft} />&nbsp;<FontAwesomeIcon icon={faGlobe} />
+    </Button>
+
+  </nav>
+}
+
 /** The menu that is shown next to the world selection graph */
-function WorldSelectionMenu() {
+export function WorldSelectionMenu() {
   const [file, setFile] = React.useState<File>();
 
   const gameId = React.useContext(GameIdContext)
   const store = useStore()
+  const difficulty = useSelector(selectDifficulty(gameId))
+
 
   /* state variables to toggle the pop-up menus */
   const [eraseMenu, setEraseMenu] = React.useState(false);
@@ -92,10 +116,34 @@ function WorldSelectionMenu() {
     eraseProgress()
   }
 
+  function label(x : number) {
+    return x == 0 ? 'Playground' : x == 1 ? 'Explorer' : 'Strict'
+  }
+
   return <nav className="world-selection-menu">
     <Button onClick={downloadProgress} title="Download game progress" to=""><FontAwesomeIcon icon={faDownload} /></Button>
     <Button title="Load game progress from JSON" onClick={openUploadMenu} to=""><FontAwesomeIcon icon={faUpload} /></Button>
     <Button title="Clear game progress" to="" onClick={openEraseMenu}><FontAwesomeIcon icon={faEraser} /></Button>
+    <div className="slider-wrap">
+      <Slider
+        title="Difficulties:&#10;- strict: 🔐 levels, 🔐 tactics&#10;- explorer: 🔓 levels, 🔐 tactics&#10;- playground: 🔓 levels, 🔓 tactics"
+        min={0} max={2}
+        aria-label="Mode"
+        defaultValue={difficulty}
+        marks={[
+          {value: 0, label: 'playground'},
+          {value: 1, label: 'explorer'},
+          {value: 2, label: 'strict'}
+        ]}
+        valueLabelFormat={label}
+        getAriaValueText={label}
+        valueLabelDisplay="auto"
+        onChange={(ev, val: number) => {
+          dispatch(changedDifficulty({game: gameId, difficulty: val}))
+        }}
+        ></Slider>
+
+    </div>
     {eraseMenu?
       <div className="modal-wrapper">
         <div className="modal-backdrop" onClick={closeEraseMenu} />
diff --git a/client/src/state/progress.ts b/client/src/state/progress.ts
index 396fc69..50ab0ee 100644
--- a/client/src/state/progress.ts
+++ b/client/src/state/progress.ts
@@ -24,9 +24,13 @@ interface WorldProgressState {
 
 export interface GameProgressState {
   inventory: string[],
+  // Difficulty: the default is 2.
+  difficulty: number,
   data: WorldProgressState
 }
 
+const DEFAULT_DIFFICULTY = 2
+
 /** The progress made on all lean4-games */
 interface ProgressState {
   games: {[game: string]: GameProgressState}
@@ -40,7 +44,7 @@ const initalLevelProgressState: LevelProgressState = {code: "", completed: false
 /** Add an empty skeleton with progress for the current game */
 function addGameProgress (state: ProgressState, action: PayloadAction<{game: string}>) {
   if (!state.games[action.payload.game]) {
-    state.games[action.payload.game] = {inventory: [], data: {}}
+    state.games[action.payload.game] = {inventory: [], data: {}, difficulty: DEFAULT_DIFFICULTY}
   }
 }
 
@@ -83,7 +87,7 @@ export const progressSlice = createSlice({
     },
     /** delete all progress for this game */
     deleteProgress(state: ProgressState, action: PayloadAction<{game: string}>) {
-      state.games[action.payload.game] = {inventory: [], data: {}}
+      state.games[action.payload.game] = {inventory: [], data: {}, difficulty: DEFAULT_DIFFICULTY}
     },
     /** delete progress for this level */
     deleteLevelProgress(state: ProgressState, action: PayloadAction<{game: string, world: string, level: number}>) {
@@ -100,6 +104,11 @@ export const progressSlice = createSlice({
       addGameProgress(state, action)
       state.games[action.payload.game].inventory = action.payload.inventory
     },
+    /** set the difficulty */
+    changedDifficulty(state: ProgressState, action: PayloadAction<{game: string, difficulty: number}>) {
+      addGameProgress(state, action)
+      state.games[action.payload.game].difficulty = action.payload.difficulty
+    },
   }
 })
 
@@ -156,6 +165,14 @@ export function selectProgress(game: string) {
   }
 }
 
+/** return progress for the current game if it exists */
+export function selectDifficulty(game: string) {
+  return (state) => {
+    return state.progress.games[game].difficulty ?? DEFAULT_DIFFICULTY
+  }
+}
+
 /** Export actions to modify the progress */
 export const { changedSelection, codeEdited, levelCompleted, deleteProgress,
-  deleteLevelProgress, loadProgress, helpEdited, changedInventory } = progressSlice.actions
+  deleteLevelProgress, loadProgress, helpEdited, changedInventory,
+  changedDifficulty } = progressSlice.actions