This is not finished, though you can still do some map editing with it. Editing mode controls whether or not tiles can be placed, and New Map makes a new map. Arrow keys to move the camera. DO NOT MOVE DIAGONALLY! You can place little blue buildings. You can drag the window on the mini-map to navigate faster.
Run this in Turbo Mode using Turbowarp. Otherwise, it won't work. https://turbowarp.org/613185188/fullscreen?turbo