Try Inkscape or Dia. Both are free. Inkscape is pretty neat and there is a lot you can do with it. A good idea is to get a screen capture program also (or maybe use Snippy?). Because you can draw something, grab it, and then insert it in to something else. (Or you can just draw something and save it.) Both of these programs are general purpose drawing programs. But if you turn on the grid and do a snap-to-grid option then you can load in tiles as you need them and apply them where you need them. Both programs also have layers so you do the ground, then the walls, then put in trees and lay them on top of the ground. Same with bushes, flowers, stepping stones, etc....