The map turned out to be buggy, and I also had a disadvantage in terms of computer resources. I had to rush and scout with a single hero. I managed to get to level 123 with a decent stack of creatures and without a spellbook. That's where I was ambushed, and I had to give up 17 creatures to buy my way out and retreat to the town. I managed to defend the castle, but I was left with only 2 units. My opponent had two mages with Mass Slow, which sealed my fate.
And even though I think I played correctly, I simply had bad luck and didn't scout properly.

Thanks for the game.