Did you click? the grasses should fade if you can build there, and rocks/plants cna be clicked on to remove them. the large turret requires 4 free squares mesuring 48x48 to be placed.
Dude! its not a finished game, its more of a test of whether people can play it, whether it lags, ect. a play test. The gameplay so far has had little ot no thought. notice the lack of a monetry system? ITS A W.I.P.!