Cross mountains and build roads over mountains.
I have to admit, I have never thought of this. Building a road across the mountain?! But wait! In the UA description it says 'units lose 50hp if they end their turn on a mountain'. So does this only apply to combat units, and not civilians?
But I am talking about Carthage in BNWI imagine Carthage could pay dividends if you take god of the sea as your pantheon (if you're lucky!) and there are lots of fish/sea resources about. Getting free harbours means you'll get hammers quickly so as long as you can keep your civ happy and growing you should have a real production boon early to mid game. Obviously - everyone else can get this benefit by building harbours but the fact they come free means you want to take advantage of it.
They're very powerful through classical and medieval and then their advantages taper off.