Incredibly deep wells and what few springs can be found are their only options, since there are no glacial-fed rivers and anything that flows over the landscape or rains get contaminated.

And I haven't gotten to this part yet, but invading them wouldn't be a bad idea. Nobody ever goes into the Ashmarch, and as such their settlements aren't fortified or guarded that well, they would be taken completely by surprise, they've managed to accumulate copious amounts of wealth unmolested by raiders and barely ever trading, and they even have their own little rebellion going on with the fire cultists. In fact, conquering this civilization would almost be too easy. Their soldiers are nothing special, so the only real fight would come from the warlocks and their demonic minions, but neither are wont to leave the safety of their giant towers.

Honestly the land itself is the main threat. Any keeper that braves marching an army through the place will be rewarded with some easy pickings.