Ruined Fortress Upgrade

[b2198]

Since Ruined Fortresses are a great way to counter siege towers, rams and close range ships, it got me thinking, what if it had an upgrade, that would improve its stats and give a small armor buff to units inside (kinda like fortified TC), and maybe even make it buildable (only after upgraded), costing something like 10 turns for ambidextery builders? And it would be useful for defending positions, since it's like a mini-TC, in the sense that builders can build from inside it, and units inside it can be targeted for buffs. The downside being that they, kinda like TCs, can be captured if one is taken by surprise or underestimates the enemy's damage and doesn't delete it beforehand.

Edit: builders can't build from inside ruined fortresses, my bad.