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An Era III technology exclusive to the Kapaku which grants Rubble Stockpiles for dealing siege damage to cities. Upon unlocking this technology, a Siege damage counter appears in the Empire Management screen. The meter accrues 1 point for percent of a given city's fortifications you damage - for example, dealing 15 points of damage to a city with 150 points of fortification will increases the meter by 10. Once the meter reaches 100, a rubble stockpile is created.
There are two things that should be noted about this ability. First, because it is based on the percentage of the city's fortification that has been destroyed, and not the raw amount, it is more effective to go after poorly-defended cities than well-defended ones. Additionally, the meter will not go up if a city's fortifications are brought to 0 and held there, so it is best to reduce a city's fortification to 0 (or just above that), stop sieging so the city's fortifications can regenerate, return to sieging, and repeat, or just conquer the city at that point.
A master craftsman wastes no materials, and this faction is no different. After a siege, shattered walls, crumbled towers, and fallen battlements are all grist for the industrial mill.