Originally developed by Robot Entertainment, but on 2011-02-24 it was announced development was being moved to Gas Powered Games. Robot Entertainment developed the core game and first two civilizations (Egyptians and Greeks) and GPG continued from there on. Apparently this was planned long ago (that Robot Entertainment would work on it only until 2011-02) with the transition being going on for 3 or more months before it finally happened.
source: www.pcgamer.com/2011/02/24/age-of-empires-online-swaps-devs/