Not bad, but...

You really need to balance your towers better. As it stands now, the only tower you should ever buy are lots and lots of Long Towers, which by rights you shouldn't be able to purchase at the start anyway, because they're the only towers that do anything remotely close to enough damage to be effective. Freeze Towers don't actually slow the creeps, and it's more worth it to flood the ground with Long Towers than it is to upgrade any of them at all. You also have a problem where when you sell a tower you can't use its space any more, and you really need to increase the amount of coin you get per kill as the levels advance.

Other than that, the simple graphics are nice, and the game runs smoothly enough. Being able to speed up the creeps to 4x is a very nice addition. I have no doubt at all that you'll take what you've done here and improve upon it in your next tower defense game, or maybe even balance this one and make it better. Keep at it!