I realise that the AC games aren't historically correct. Some of the AC:O stuff is so far from reality it isn't funny (Kassandra would have been killed for just trying to watch the Olympics, let alone attempting to compete in them for starters). I'm more worried about the message that the trailer is giving - that resisting an aggressive colonising invader makes you the bad guy. Probably not the message that you want to be giving out.
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The game is a couple of centuries too early for massive castles. Castles came along with the Normans when they conquered England, and even then it was fairly simple motte-and-bailey designs.