I have gotten to the second level og the undead campaign but even after clearing and capturing every building in the whole map the game won't let me exit through the west gate to the next level. Must be a creature hiding somewhere but it has always been like that I can't seem to find it. Any ideas?
Or should I clear everything including the neutrals?
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You have to defeat the AI, Purple and Green, i.e. capture all their towns and defeat all their armies. If you're sure you've done it already, check the mines (some have Purple troops) and flag the garrisons too. If nothing helps, try reloading the last save and end your turn. I remember such a bug in singleplayer maps.
Btw, have you completed the quest and got the sword? It's not mandatory but it is a carryover artifact.