A nice start.

Graphics: 3/5
They were ok, but they could do with some work. The text and most of the sprites weren't too good.

Sound: 1/5
There was none. Period.

Gameplay: 3/5
The AI is well functioning, although they could use more work. Placing towers was ok, but sometimes you could place certain towers on the concrete, which you said you can't do.

Story: 1/5
Not much of one, as of now.

Interface: 3/5
Well, the wave (not levels) bar at the top was somewhat nice, even though it moved very slowly and didn't look too smooth. There is no menu and at first you have no idea what you are doing. There should be a stat bar at the bottom so you can tell how many enemies are left, etc, or have a help page where you hover over to bring up the stats of each thing, how to beat it, etc.

OVERALL: 3/5
It's a nice start, and with some work could become better. Make the waves random, and make them come on faster. Add sound to everything and fix some of the bugs and interface.