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So what if it's set in the Middle Ages? Cannons are fine, and ballistae are debatable, but at least crossbows/scorpions could be used. Plus, adding tactical abilities like fortifying terrain, trenches, etc., would be great. And traps... Siege weapons like ladders, too. That's just how I see it.
No, regarding priests, I was more thinking about skills that simply slightly change the effect depending on the religion. For example, all priests have healing. Some priests, when healing a character, can give them a fire shield, others heal even the dead, and others temporarily increase a certain parameter, etc.