If I purchased the game as a CD from a store, is it possible to register the game on Steam? I bought a physical copy of EUIII Complete, but now I am interested in purchasing the HttT and DW expansions through Steam. I tried to register the game on Steam by simply putting the CD key that came with the game into their system, but it says it is an "invalid CD key". Will I have to buy the Steam version of EUIII: Complete if I want the Steam expansions?
Steam won't let you even buy HttT/DW if you don't have the Steam version of the base games. Best to buy via gamersgate, you can use your CD Complete, and there are less problems with patches/MP/everything.