You can learn a lot from playing one of the billion flash TD games out there, obvious things you need to add:
* At the very least, make it so the first wave only begins when you press some button. I didn't have time to read all the tower info before the wave started.
* Add a button to instantly begin the next wave
* Make it so holding shift allows you to continue creating buildings of that type, mashing number keys is a bit annoying
* Redo the graphics, and make the enemies rotate more smoothly
* Add some actual maps, currently mazing is highly boring as it requires no thought
* Being able to build over the spawn/exit makes no sense
* Add word wrap to all the text fields
* Bind pause to p as well
* Display the gold somewhere else(maybe at the top? you don't need that much space for the level), as you can't see how much you have when you go to upgrade
* Use better text, when text overlaps you just get a block of yellow which is rather hard to read
* The start of the game is highly boring, all you do is construct the maze very slowly because buying so many arrow tower takes so long, other games give you a rock or boulder tower which costs almost nothing so you can quickly make a maze then add proper towers
* Building a tower that obstructs the path should be disallowed, allowing it to be built, checking it, then selling it is kind of stupid
* Why does gold appear to deduct before I've placed a tower?
* Why is the enemy selection box thing totally off? And why does it change size when they turn?