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I didn't play it on 1.6. In 1.5, the computer attacks on the first night. The second attack is on the 2nd night, meaning there isn't enough time for mana to regenerate. Therefore, on the first night, I put Shadia in battle, and Raileg took the Castle.
In 3.0, I captured the Castle with Shadia on the second day, moved her as far back as possible, and Raileg entered the Castle. I managed to complete it thanks to the bug exploit with summoning and invisibility, as described above. In 3.0, the computer has significantly more troops than in 1.5.
The losses were significant. But I saved the Furies because I didn't put them in battle. They are not for street fights.
The second city should be captured on the last day of the week, buy troops, and run away through the southern portal.
Everything described above applies only to the Hero difficulty level. On Veteran and lower, the second enemy will remain in the Castle. Therefore, it's pointless to send Shadia with a couple of Hydras there. On Veteran, I captured all the mines around the city in the first week, and captured the second city on the last day of the 2nd week. After that, it was easy. Shadia and the purchased heroes defended the captured territory, and Raileg captured new cities.
In 1.5 on Hero and 3.0 on Veteran, I leveled up Raileg's defense, chaos, education, magic, and luck. In 3.0 on Hero, instead of luck, I leveled up summoning.