Scale here would be very important to determine the best placement for defensive structures. Important considerations when creating a defense are the capabilities of the defending forces as well as the capabilities of an attacking force. If the defenders have trebuchets, which would be limited in effect against a moving target, you certainly have more range/distance to work with as far as a defense goes. Arbalests and ballistae would have a much shorter range, but more functionality as far as defense goes. If you have magic casters throwing fireballs and the like, they're range is also going to be limited.
On the other side of the coin, the harbor you present is rather easily blockaded from the sea, depending again on the capabilities of the aggressors. A fleet of ships with a handful of corsairs could keep the city landlocked for quite some time with adequate supplies and logistics. If they have trebuchet mounted on their vessels, they could park and lob things at the city at will, given adequate ammo (barges loaded with boulders?)
Some things to consider as you plan the defense of this city are additional natural features, such as a large coral reef a bit off the coast, perhaps one with a labrynthine arrangement that only the local sailors know the path through and is a very closely guarded secret by the local lord/guilds/military. Michael Moorcock built a maze into the walls of Imryyr in his Elric books that only one person knew the way through, so if this is for an RPG, something to consider. Perhaps extending the sandbar or the erection of a seawall, depending on ocean currents and winds? Some strange magical catastrophe that formed some mighty rock formation? There's any number of ideas, but consider the history of the city, the region, their traditional enemies and the capabilities of any potential forces in the conflicts you intend to forestall and that should guide you in fleshing out the map. Also, if this is for an RPG, it might germinate some ideas that you'd never considered!
It's a great start!