You also should getting into the habbit of adding a level name prefix. This will help if say you want to have separate tilesets later on for inside/outside/event levels without having to set the tileset in each given level. I wish this were a requirement on testbed.
example:
PHP Code:
addtiledef("mercia_newtileset.png", "myworld_", 1);
Edit: it's more or less personal preference I guess, but I'd start doing it.