Demon invasion.

Well, it's just unusual and inconvenient when heroes can't enter a castle in the usual way. Otherwise, the game progresses normally; you just have to take a Demolisher and train other units so they can use a spell to enter the castle. A year and a half ago, I completed this campaign, and I had EWA+NHF installed at the time, and this didn't happen, so I was surprised when I discovered this.