I had this on Atlantis as well, it happened when the defensive towers from the side portals weren't killing mobs fast enough (fast spawn+50 mob limit). They would focus on the minotaurs constantly, which allowed the archers to build up until they closed off the minotaurs. This happened before the cataputlisks were spawned, or the priests, so I guess war stage 2-4 somewhere. The enemy had the warlord flag which adds extra units to every portal as well.