Eateng-san does have a point: historically, constructing castles and the like took a long time and required lots of manpower. Remember, the construction technology we're talking about here is basically at the level of the ancient Egypt (no wheel, mobs of semi-slaves dragging around multi-ton stones). Of course, it would be an improvement to be able to divide your resources to construct two buildings simultaneously (which would require twice the time for each), but that kind of thing would be useful only very rarely.

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"When you leave your gate act as though an enemy was in sight."
- Japanese proverb