Thanks for the feedback! This game was developed over an extremely short timeline (2 months!), so I didn't have time to put in all the polish I wanted to. Earlier on in development, I had a victory condition of "destroy all monster lairs". However, the end game would drag on as the heroes had to explore every corner of the map to find the last sewer or seedy tavern. I changed the victory condition to be "destroy all of the highest-level monster lairs" instead. As you note, I don't make that clear anywhere.
I'm hoping to expand the game for a commercial release, and one thing I'll take a look at adding is multiple victory conditions. For instance, maybe one map requires you to build and defend multiple guild halls from waves of rampaging monsters. Maybe another has you preventing a second apocalypse in the city. Let me know if you can think of other cool ideas :)