Gauntlabo was a project to create a fun little Gauntlet-style game for the GBA. Although the project never really got off the ground, mainly due to other commitments for the group members, we did create two map editors and quite a bit of concept and tile art.
I did one of the map editors and a GBA stub to display maps, mainly to get the artists started. The editor isn’t very complicated, you select a tileset (stored in a free-form bitmap), and define your metatiles (including HV flips), although they are saved in the map file itself. This isn’t really a bad thing, since you can always copy a map with decent metatiles defined, and then clear it, but it’d be one thing I’d change.
The editor has 2 standard layers, and it lets you paint properties on any tile (although there were only two properties in the game engine at the time). It has all the code to place and save entities, but no entities to place…
Feast your eyes on my programmer-art tileset (yes, that green blob is meant to be a bush)