My advice:
Research the pellet gun and upgrade it to full
Research explosives, then the grenade thrower up to full
Research the Barrier
Before finishing the second level, make sure you have at least $10,000, or start the game over. Otherwise, you will never survive the last level.
Place your towers as follows:
Beginning from the rear, four pellet shooters, fully upgraded, then a grenade thrower, fully upgraded, then two barriers. Once you have all that built up, you will spend the level replacing barriers as they fall, but none of your men will be hit.
easy peasy
you can then research/upgrade everything, for fun, to get in-game achievments
waves are too long, upgrades too expensive, and the attack power of each succeeding wave is too great to make this game enjoyable
design and concept are well executed, 5/5
due to gameplay problems, I rate it overall a 1/5
I think it is a really nice game. My only criticism is that sometimes the sounds do not mix well, for example, the guitar soundtrack plus the entrance of the king, since it is also musical. You could just get rid of that, and the gameplay would not be changed, but we would not be so jarred each time.
:(