I don't think I placed a Fortress. I haven't sent the new version to anyone yet, but in any case, the green upgrade is final and doesn't transform into anything else.
Green reinforcements are sent, and this upgrade isn't offered in the castle. So, in fact, they are extra troops that aren't useful for anything. Therefore, I'll play without the green faction – I'll convert them into standard units. I still don't like 2 of their units – the Knights (the Zealots are better with their 2nd attack) and the Griffins (I prefer the Sky Riders; the 7 speed from the return ability isn't very useful, in my opinion).