Quick question -- Walls and Castle give a defensive bonus, except against "gunpowder units." I've always assumed that gunpowder units in this context included cavalry, since they can't be built without the gunpowder advancement. On the other hand, they're listed under the military adviser as mounted units, not gunpowder units. So just wondering what walls and castles coverd. Thanks.

Any unit requiring gunpowder counts, so cannons and cavalry (and frigates as well).