Incredible!
This is definitely one of the best games I've ever played. The concept is amazingly original! Even with my crappy computer the graphics ran very well and I hardly had any lagging. The entire system was a work of art. I'm glad there was a limit to how many you could summon because that made it sort of a puzzle game as well as a tower defense game. The upgrading system was probably the best I've ever seen for a flash game as well. It was difficult to make enough money for those really good upgrades, but definitely not impossible, which is great. Each unit you could summon was original, and the individual limits that were on them was interesting (in a good way). The only problem I had, and I'm saying the ONLY problem I had with the game, was that the ending was kind of anti-climatic LOL But other than that, perfect in every way. All in all, this game was amazing.
Also, I thought the secret achievement was kinda funny.