Made creatures spawn according to configurable conditions

I've used http://forum.freegamedev.net/viewtopic.php?f=37&t=5851&sid=e4b5e9f864223198e7338b4a0061c065

Tier 1 creatures need nearly nothing to spawn (except goblin the forge).
Tier 2 a little bit more (mostly dormitory)
Tier 3 rooms and hatchery
PitDemon : many rooms + 2 dragons (you won't see him as easily as you are used to ^^)