People were complaining in the beta about cities being sacked by monsters. It is more likely to happen on higher difficulty levels (so it does happen). I would like the monsters to be more aggressive, but when I play on challenging its pretty tough anyhow

I recently did a game where I expanded too quickly. My third city was sacked by a horde of 4 spiderlings and one spider. The sand golem plus garrison only managed to kill 1.

Now I have a quick reaction force of mounted troops that can quickly travel to an endangered city by street when monsters appear in sight.

Difficulty Challenging of course.

From my understanding, monsters go after units, but only hit settlements and outposts at random.  If the settlement or outpost happens to be in the way, then they will hit it.  In the last Beta I was hit by a monster.  Of course it was a Slag, so that pretty much sucked.  It was my third city, and it was around turn 35 in the game.  They could not handle the beasty