It's my fault; I ignored the main hero's defense, took the castle too late, and simply didn't have enough money to buy out the hero and the ballista.
I'll replay with the "fight to the last" option, and I'll try to take the castle and build the Capitol faster. I didn't see the mentor nearby, but it's sad without the "fight to the last" option.
I chose the starting bonus – the tunic – which was the right choice; without it, there's no chance.