The Moveto+avoid function makes it possible, by using only one object in the playfield; towers. All creeps plan their paths avoiding towers, and the entire playfield is surrounded by dummy towers to keep them inside.
I left the actor FINDR with normal visibility state, so as to de-mistify this game method for you. In this type of tower defence, you cannot block the path entirely, which is what makes it more difficult to program, because you have to be able to assess the path before placing the tower. You will see a white spark fly across the screen whenever you attempt to place a tower; This checks to see if your tower placement is legal or not.
Everything else is pretty normal, there are bugs-
-Creeps 'cheat' sometimes (problem with moveto function?)
-Upgrade timer works strangely
-Stat 'name' doesn't strcpy correctly
Anyway, that's what you can do in 8 hours with the incredibly free toolkit of Game Editor + Inkscape + Ferals sprite rotator.
Enjoy!
I'll try it later
