That sounds a tad too complicated. Maybe we should simplify it a bit.
1) If one commander can build a castle, normal workers are probably available anywhere.
2) Thus, the units that actually build the catapult are available everywhere.
3) We only need a commander. This commander could be named "Master Catapult Engineer" or "Siege Engineer".
4) Every Siege Engineer should give a bonus for breaking down castle walls. I think 25 would be a good value. Siege Engineers should be able to lead few units, which can emulate the warriors operating the catapult or just guarding it from enemy attacks against it (assassinations). If you really wanted to play around with the idea, I quess we could give the Siege Engineer no fighting equipment, but high resource cost, like 50 or so. And this is very, very Ulmish; they already have Sappers and Guardians and Lord Guardians and - oh, Siege Engineers already exist!
Isn't it nice when the developers are ahead of us?
