Is it possible to spend a few months on the first island, upgrading castles and building an army?

Yes, you can. Last time, in version 1.5, I spent several months doing just that, until I got all the high-level dark magic and summoning spells. There's no point in staying just to build an army – Markel should be able to defeat everything that moves (and doesn't move) every week.

This time, in version 3.0, I spent a long time in the previous mission to get a complete set of dark magic spells. As a result, the enemy's last castle had 65 titans and everything else. But for my super-upgraded Godric, that's not a problem. I'll learn all the summoning spells in this mission.