Why would MertBayar risk being caught cheating just to quickly pop up a couple of unnecessary stables when he actually needed to quickly build houses a few minutes earlier but didn't use the "hack" then. Furthermore, if he was going to blatantly use an aegis-style cheat (I say blatantly because those stables were built right up front where you could see them) why not just use it at the start to gain a huge edge over you that would enable him to pull off a nice rush? This is almost certainly the result of a bug, something I'll try to re-create/trigger later today. It would be helpful, though, to look through this player's recorded games to see if he's done anything else like this before.
That's slightly incorrect. It's actually if a villager gets hurt by an AoE (area of effect) attack.L_Clan_Road made me a demonstration of that bug while I was cooping him.
He ordered a mangonel to attack ground near his vills, than they build a castle almost instantly
So i guess it works with any siege unit hitting a vill...