I was playing a map, when the siege began. It was around day 10 in the game. Appeared some slimes and had no problem. One more time later on the same map, the more the 15th of the 2nd month, a new siege with some normal spiders.
I started another map. And I got an army of giant spiders in just 8 days. they simply swept all the villagers.
Is there a standard for siege or level of which is random?
Playing on 0.45b

Stupid people are dangerous in large groups. Especially when we live in a world with water pockets in jungles.