CanvasQuest Editor (The Magic Canvas)
pick a set
Choose a tileset from the dropdown list, then click on a tile. Click on the grid to place tiles. Check the collision box to make the tile a wall. Placing monsters work via the Monster tab. If you want to make a treasure item, you have to place both a Treasure Item, and a treasure Event on the same grid element. Look at existing levels for an example. Copy paste the code you create here and save to a js file somewhere on the net. To load your level, start a game, get to the tiles view, then press "l" or "L" (depends on the browser), then type the full URL of the level you created. Good luck creating new adventures!