Castrum - Sony's first-party studios (and second-party as well). This has also been the key to Nintendo's success.
If Microsoft had been smart, they would have realized this and begun buying up developers left and right at the dawn of this gen. Instead, they offloaded Ensemble, lost Bungie, gave up on buying BioWare before EA did, and handed the most overrated game developer of all-time, Peter Molyneux, a big fat cheque so he could continue churning out Fable 1 with minor improvements every couple of years.
Also note that FASA Studios, of MechAssault fame, was similarly killed by MS.
http://kotaku.com/5827180/just-how-many-game-studios-does-sony-own-anyway
