Ranger – “Defense” [Campaign 5, Mission 3]

Ugh! I spent the whole day trying to complete a mission, and now there's a huge glitch. First, I left Ilfina in the castle, and then Fedor took the archers, druids, and unicorns. With that army, he cleared all the blue castles and garrisons. The purple one just stood there and did nothing. At some point, another guy appeared next to the dreadnought. I even saved the game before the first dungeon and gathered all the prisoners. I didn't get any missions about the dungeon or the garrisons at all. I decided to go and kill the purple one, but I couldn't even get close to him; it was like an invisible wall. I reloaded before the dungeon, and there was the same problem. Do I really have to start everything over?