The veil is lifted.

I haven't had much time lately, but I've started playing again... So, a month and a week have passed, I captured a town, completed the campaign for the region, and killed the mage. And only then did I realize that one of the missions is time-limited. Question - will I be able to complete it, or should I start over?

You'll have time, you have 3 weeks left. Destroy the army near the shipyard, build a ship, sail north first, and then, at the fork, southeast. On the southern bank of the river, disembark and head east. There's an army stationed there, consisting of Adepts, Archers, and Fallen Angels. Attack. If you win, the mission is complete, and you can leisurely improve your skills and slowly prepare for an attack on Waterway.