First let me say I love this game. I would like to let the person who made it know of some bugs though. When you sell an item such as a normal gun it says you should get $3 BUT you only get $2. Also As it was pointed out the enemy that spawn REALLY SUCK because of jumping over walls and in the last mission they are impossible to beat. I had about 5 of them all ready to die...then they split right at the end and I lost. I understand the need for a hard enemy with high hit points...but that many?
All that being said let me give some people a hint. When you set your towers up do not think in lines but rather in stairs. It forces them to move slower as they make their turns after every tower. Also UPGRADE YOUR GUNS! But even with this you need to remember to upgrade the ones at the end of your maze first and only upgrade one at a time.