Demon invasion.

Here’s what happened when I was playing a mission outside of the campaign: I didn’t have enough gold because the castles had limited resources (the only unlimited one was the Knight’s castle, but it’s not a viable option, and I couldn’t even reach it). All the heroes were carrying bags, so I had to change their equipment every day. So, you can’t really summon many troops, and on top of that, you need to summon troops from the Necropolis castle. When you transition to the Necropolis castle, the garrison seems to fill up with a huge number of spectral dragons that can’t be killed. So, without summoning reinforcements, the troops that arrive via caravan at the Necropolis castle can’t be accessed, which would be very unfortunate.