Build times are fixed. The only purpose served by having multiple commanders build a fort is that you won't lose your investment if one of them gets magically sniped.
Troops do not help at all. Soldiers are paid to stick pointy bits of metal into other people, not lift heavy stuff. That's what God made peasants for .
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Anything wrong ?
Blame it on me - I'm the French.
Thank you kindly. It's not the most intuitive, but I guess having huge armies insta-building everything would break the balance of the game.
I still think that minimum build time (fort no sooner than 2 turns, for example), plus penalties for having small number of builders would make more sense. After all, all buildings required manpower in history, but I guess you can tell it's abstracted by having the commander force the local peasants into work.