In all of the Mak campaign that this is in, now at 181 BC, I have never, ever seen this building available. So what made it available there? The only thing I can think of is that there was a Roman City Garrison and my Mak Forward Garrison. But damn, the pic looks like a friggin' castle!
So, just what IS it, and what build tree does it belong on? And why have I not seen it until this far into the game? Makes me wish it had its description and art done, cuz now I'm one curious monkey (no men with yellow hats need apply).