Great Barracks

A second infantry queue, running alongside the Barracks rather than replacing it, in any village that is not the capital.

• Troops trained here cost three times their normal resource cost.
• It runs its own queue at its own level, so both produce at once and a village turns out infantry twice over. Build it once the ordinary Barracks is already high, not instead of raising it.
• It is a hammer building. Paying triple buys concentration, and concentration only matters when the army has to be in one place - which is what a Wonder world asks for, because troops cannot be merged there.
• Where merging is available, training across several villages and merging later does the same job without the surcharge.
• It is almost never right for defence: defenders can be trained anywhere and walked in, so paying triple to make them in one village buys nothing at all.

The maths. Training time is the unit's own base time, cut by a tenth for every level of the building: base x 0.9^(level - 1), divided by the world's speed. A Romans Legionnaire takes 0:26:40 at level 1 on a 1x world, 0:10:20 at level 10 and 0:03:37 at level 20. Because each level takes a tenth off what the one before it left, the saving compounds - and the last levels are worth less in minutes than the first, even though they cost far more. A Great building runs its own queue alongside the normal one and counts its own level in that formula, so the two are independent - and it turns out three units a batch rather than one.