whoop, just saw this.
so this is more of a map-making question than a hacking question, right? you're doing it in Mappy or whatever, I'm guessing? sadly, i think a lot of it is just trial/error and looking around, but generally speaking, somewhat similar stuff tends to be grouped together... but more than that, I think you can get a good idea by looking at existing maps.
for instance, I'm not sure what you mean by a fort (something like chapter 4 in FE7, or chapter 7x in FE7?) but by looking at a map that has the same tileset or the like, you could see which tiles you need, then look for those in the full tileset, and go from there.
if you didn't already figure it out, hopefully that helps a little