For example, Castle + Fortress = Castle?
City + Town = Castle. Castle + Wall = City?
Metropolis + City = Megalopolis. Ok we're getting somewhere cool here, let's see how big it gets: Megalopolis + City = Metropolis?!
Finally, it just failed to combine War + Tunnel. It blinks for a minute and then gives up. I would have said "Sappers"? Edit: There are actually many such failures for higher-order combinations which is strictly not infinite. Other combinations described above might technically fit the bill but ceasing functionality does not : /