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You can reach the sanctuary in 2 turns through a portal, then proceed to destroy the garrison. I destroyed the garrison of the city of Erish three times: once with a Markal without a key, then with a Sigrif just for practice (the auxiliary heroes took down the Markal, but in that turn, there was an opportunity to destroy the garrison with the Sigrif; I destroyed it first, then finished off the Markal), and the third time, I destroyed both garrisons with the Markal. It's true that he's quite powerful, but if you get on a ship, the probability of being caught increases.
Encountering Sigrif = defeat; you can't even freeze him. But you can attack the auxiliary demons – if you catch them. I only managed to catch Alastor. Deleb transferred troops to Sigrif, accumulated necromancers, but I never caught her; she always jumped into the portal to the city – the cities are inaccessible.
Regarding the term "prolonging the game," it's a strange word. I won on the 10th month, 4th week, 1st day, and nothing happened. Even though it was impossible to defeat Sigrif with Markal in a direct battle.