[Edi]

I should also add something which is not apparent from my previous post: This AAR game is now progressing somewhere around turn 80 or so, well over six years into the game. Most of this time I have had a rather tenuous control over the monster lairs, especially the brigand lair, which proved so troublesome.

The ancient forest near my starting citadel has been notably quiescent for monster lair and the occasional stack that has emerged from there has generally either wandered off or been slain before it had a chance to do much damage.

Going into completely unexplored areas where there are ancient forests, brigand lairs etc. after such a long time, one would be far more likely to run into multiple armies of independents and one would expect a fairly intensive stretch of combat until the area was cleared.

The easiest way to prevent big problems with independents is to act aggressively against monster lairs and flag them, then leave a sufficient garrison that it can't be casually swept away. In case of ancient forests, if you can't flag them, either acquire a pyromancer or a fire breathing monster and burn them down. Enchanters can also turn them into golems, which will likewise destroy it.