Thank-you, LoneTraveller, I was more interested in specializing cities even further, AND eliminating the need to scroll through 30 or so units in the city build menu. Perhaps an Early Tank factory, and a Late Tank Factory. That way, you could specialize your cities to build just ships, just armor, just artillery, just aircraft, if you wished. But is there a limit to how many units can have a building as a prerequisite? That is my real question.