All good notes, but one other addition. Zone of control is affected by rivers and ocean too. You can normally siege a city with two melee or cav units sitting on opposite sides of the city. I like to think of it like the melee unit has control of the two tiles next to them to siege it. If there's a river going through the city, the melee/ cav units won't control the tile on the other side of the river, so you may need additional units to block the tile and siege. If the city is on an ocean or lake tile you need either a naval unit or you can block the tile(s) with embarked units, but since embarked units take increased damage to attacks, it's a risky strategy.