great game. really did bring me back to those old TDs. I did like how you made it rapid fire, but i have to say that some of the later levels were incredibly difficult, and that the AA seems to be very weak. consider adding units made for fighting flying, cus some of these levels were way too hard because so few towers attacked air. The difficulty wasnt really a bad thing, but it did make me really learn how the game worked, so i guess in that sense it does open it up for sequels/ more level or tower packs.
^^^those are just some thoughts. my real issues with the game were that the prices for upgrades were frequently innacurate (one of them says 10 every time but costs 250), and that the personal upgrades did not build up over time (they were so small that making banks or science buildings would do more, and i ended up with tens of thousands of research points that went nowhere)
Great job, i hope i didnt sound too critical. :)