MonsterSwarm mod enhances the AI of Minecraft mobs. Monsters can attack from far away destroy walls on their way, build bridges and kill your animals.
Reasonable default config:
Monsters target you from far away at all times.
Zombies/Skeletons can digg only underground (below 40) in overworld
in other worlds they can digg everywhere.
So you are safe in your base on the surface in the overworld, but anywhere else is dangerous.
This mod changes blast-resistance from some blocks. In general, bricks are a bit more resistant (works with bricks from mfr, railcraft, bop), and obsidian is nerfed. (it would be op)
Config file is pretty basic, but self explanatory. Use commmand
/swarm reload to reload.