So we all know the AI loves to make a city on a 1-tile island. If I wanted to attack that city to capture it, how exactly do I go about doing so, being that I can't land any units next to it?
Just give some of your land units the amphibious promo, bombard the city with ships and then send the promoted land units in. Well, I say "units", but you could probably do it with one amphibious unit and a few ships.
Firaxis removed marines especially to make capturing such cities even more difficult.
You DON'T need the Amphibious promotion. It does make it easier, but it is unnecessary.
Depends on the city and on the tech level. I've run into some situations where it's a suicide attack trying to take that city even though it's fully reduced. The Amphibious promotion has helped quite a bit in those situations
Aside from that situation, though, those cities can be some of the easiest to conquer due to the AI's inability to defend them.