Sweet
One of the best defense games I've played. I liked the graphics and the variety of enemy sprites and strengths.
Yeah, I have to agree with the other people that quad cannon is pretty useless unless you have enemies coming from exactly 90 degree angles, which would spare you 3 clicks, and which doesn't occur frequently.
Rather than making the levels random-ed a bit, try making a few levels where enemies will line up exactly the 90 degree angle so that players will get a feel of the quad's importance, because in most stages I can wipe out the entire wave using one cannon and weapon 2. Like how some of the levels have a suicide plane strike from all directions, which demonstrates the importance of weapon 4.
Weapon 3 is a luxury weapon. It drains too much energy. You can offset this by maybe affording the player more energy capacity per upgrade. Significant improvements on the capacity, I mean.
Overall, I really enjoyed the game. Those spider enemies remind me of a certain unit in Total Annihilation (PC game).