Boris K
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In my opinion, the easiest way is to quickly capture all the castles without getting distracted by anything else, and then look for obelisks and level-up locations. After capturing all the castles, you can lock the auxiliary heroes inside and forget about them and the castles themselves. Large armies are not needed to clear level-up locations, obelisks, wizards, and dragons. There is no need to clear anything else. Even mines and chests are not needed. There is no one to fight. In my last playthrough, I didn't even take the key because I knew it didn't open the way to level-up locations or obelisks. And there is more than enough experience to reach level 18 in this mission.
To block a castle for many months, you need to replenish the army of the blocker (but I'm too lazy). This requires mines and money. The probability that Shadi will immediately get all the dark magic spells is very low. In my last playthrough in version 3.0, I even had to replenish Railag's spellbook. There was no stopping cold in any of the castles. Of course, Shadi also needs a full set of chaos spells.
To block a castle for many months, you need to replenish the army of the blocker (but I'm too lazy). This requires mines and money. The probability that Shadi will immediately get all the dark magic spells is very low. In my last playthrough in version 3.0, I even had to replenish Railag's spellbook. There was no stopping cold in any of the castles. Of course, Shadi also needs a full set of chaos spells.