Sanctuary – “Crimson Tide” [Campaign 4 – Mission 3]

For some reason, there was a catastrophic shortage of money in this mission.

That's definitely true.

You will be able to convert cities when all four have been captured.

But I managed to convert them when only 3 out of 4 were captured. When I converted one of the cities, a disgruntled orc immediately appeared and started interfering with the fight against the monsters, so I replayed it a bit and started converting after capturing the 4th castle. :)