It is essential to visit the teleportation locations and destroy the Inferno Castles underground as quickly as possible!!! - otherwise, their growth will outpace yours. Considering the forts and external structures, it's about 10 times more. :)
Controlling the entire surface and only then going underground is not the right strategy. It definitely won't work on Hard difficulty.
The enemy only attacked the lower castle on the left on the 3rd week; by that time, I had already finished the upper part and jumped back to the first castle. I didn't experience any problems with being attacked from two sides.
At the end, I exploited the AI's stupidity - when approaching a castle with a large army, I eliminated the hero at the gates and ended the day. The next day, the computer bought a new "gatekeeper" and shared a significant portion of its army with him. After 3-4 days, I could capture the castle with almost no losses.
I did this solely to save time and nerves, using auto-battle; by that time, the main strategic tasks had been solved, and I had more castles and forts, so I didn't want to waste time gathering troops.
I only skipped days before the battle with Uriel. I didn't manage to gather enough money to buy back the entire army.
However, I accidentally obtained the "Celebration of Life" achievement - not killing anyone during the festival week :)))))))))))))))))