Pretty nice game, got to level 102 by chaining lasers. The trick with them is to keep firing speed in sync - if you can't update at least four laser tower firing speed before a wave hits, don't do it. Only upgrade range on a very few "edge" towers, since the rest will only be chaining to the nearby neighbor.
The thing that really annoys me about all tower games I've played is that the targetting AI is lame - imho it should always be the farthest advanced unit that is targetted.