I played this game recently http://www.kongregate.com/games/JuicyBeast/knightmare-tower and I am amazed by the way how different monsters are beign spawned.

I personally developed my own shooter game and I added time based but also count based spawing system. By count based I mean when there are 5 enemies on stage stop spawning. But this is one example.

My question is how are these spawning mechanism built, is there some pattern or some theory how they are built? Are there some online materials/pages where I can improve my knowledge?

To sumarize, let just say we have 6 types of monsters. I start the game and kill of monsters of type 1,2 and 3 all the time. Once I pass the first ceiling, like in the game above, monster type 4 appear. ANd so on. As I progress trough the game, the same system of 6 types of monsters stay, but they become more and more resilient and dangerous. So I must also improve to be able to destroy the same monsters but now stronger.

My question is simple, are there some theories built or written for developing this type of inteligent systems?

Note: This is a general question, not tied up with some game or how exactly should the game work.

I am capable to program my own mechanisms but I think I need some help. Thanks.