I've been the defender on quite many sieges, there was 1 siege where AI for some reason refused to send its archers to firing range (3 units) when I had 1, so I had to go and get them. Though at another siege AI sent its general to firing range of my archers (6 units this time) and just stood there, but this was late in the siege when enemy only had 1 other unit not routing. Now that you need generals around to keep morale up, AI seems a bit reckless and throws them to combat (not directly charging spear units at least).
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