7.1 — Tilemaps: The Level Editor

Project window → Assets/_Scenes, and open Platformer.unity. Game view → press Play and walk: you can move, you can jump, and you stand on rock. Now try to change that rock. Hierarchy → select things until you find what the floor is made of. You will land on a Grid with three children under it — and the middle one is the reason this hour is about choosing which tilemap you paint into.

Object	What it is for	What it already has
Grid	The snapping contract. Every child tilemap snaps to its cell size, and every system built on top of the level inherits that spacing.	A Cell Size of 1 by 1, and three tilemap children.
Collision (child)	The rock you stand on. 75 tiles, filling a box roughly 16 tiles wide and 10 tall — a cave shape, not a solid rectangle.	A Tilemap Collider 2D, a Composite Collider 2D, a Rigidbody 2D whose Body Type is Static, and the tag Ground. Physics already works. Leave all of it alone.
Cave Exit (child)	The way out. Two tiles, at the far left of the floor — the smallest solid thing in the scene, and the one everybody forgets.	The same three components as the floor, and Untagged, because it was built for a different job: SceneSwitch.cs sits on it and sends you to the Overworld when you touch it.
No Collision (child)	Pure picture. Backdrop, decoration, anything the player should see but never bump into. It holds 85 tiles — more than the floor does.	A Tilemap Renderer, and nothing else. No collider of any kind, so nothing here can ever stop you.

So the level editor already exists and the physics already works. What is missing is everything you need to keep building: the palette — the tray of tiles you paint from, and the home of the art it is made of.

1. Project window → Assets/Artwork/Platformer/Sliced/. Everything this project owns for the cave is in there. CavesTileSet/ holds the cave sliced into individual Tile assets — 1,829 of them, each a small .asset paired with a sprite. CaveEntranceTileSet/ holds the cave-mouth pieces, 250 of them. The two prefabs beside the folders, Caves and CavesEntrance, are Tile Palettes with those tiles already arranged in a grid, and each carries a Palette Settings component that says so. The raw source sheets live in Assets/Artwork/Platformer/OriginalFiles/ and MainLev2.0.png, for when you need to re-slice. There is no Assets/Sprites folder and no Assets/Tilemaps folder in this project — if you have been told otherwise, that was for a different project.
2. Window ▸ 2D ▸ Tile Palette → the palette selector, and open Caves. Tiles fill the window. If you would rather build your own tray, create a new palette and drag the Tile assets out of CavesTileSet/ into it — the .asset files, never the raw PNGs, because the slicing is already done and re-slicing a finished sheet is a job you take on deliberately. Save a palette you make yourself in that same folder, beside the art it uses, so it travels with the project.
3. Hierarchy → select Collision before you paint rock, and No Collision before you paint backdrop. Watch the Hierarchy highlight to confirm which one has focus: painting into the wrong tilemap is the most common first-hour mistake in this unit, and it produces a picture that lies to you.
4. Tile Palette window → learn the five tools: Paint (drag to draw), Rect (fill a rectangle), Fill (flood a region), Erase, and the Eye Dropper (grab the tile already under your cursor). Scene view → paint a six-tile-wide ledge jutting out of an existing wall, and a two-tile pit floor below it. Then erase half of what you drew and redraw it — the point is the hand, not the level.
5. Scene view → extend the floor to the right by ten tiles, then Game view → press Play and walk onto your new floor. If you can stand on it, you painted on Collision or Cave Exit. If you fall straight through it, you painted on the third grid — and now you know exactly what "No Collision" means.