Walkthroughs for Campaigns and Maps in WoGA.

You can complete the map by destroying three more towns. It is better to start with the elven Castle (take it using three units from the town behind the garrison), then hire snipers in three locations (they cannot be used against elves — they refuse to fight). With the snipers and peasants (who are plentiful around the garrison), clear out the swamp (and thus destroy the town). You can answer the Hydra's questions about whether you believe him or not however you like; he won't do anything (before the elven town, you are offered a potion, but I didn't understand what it changes). After destroying two towns, go south from the swamp and look for the monk, who says he won't let you pass until you do a good deed. After visiting the monk, the forest near the Necropolis disappears => go and destroy it. Return to the monk and you win.

The problem is reaching the garrison in time. I manage it once in a while; simply when you sleep near the statue, the number of days changes by 3 (before capturing the Castle), so you need to find the shortest path.