First I'd need to know where the assault was coming from. There are different defensive tactics based on whether the enemy was attacking from land or by sea or even in a combined operation. Also knowing what type of siege equipment the enemy would bring to such a battle would be important to know, e.g. catapults, trebuchets, siege towers, ladders, etc.

Another thing to keep in mind is the number of enemy soldiers as well as the number of men in the garrison for this settlement. If I have a sufficiently large garrison force in comparison to the enemy force, I'm more comfortable fighting on the walls and spreading my forces out. However, if I have a much smaller force than the enemy say a 5-1 or more advantage to the enemy, I'd focus more on defending choke points in the settlement to offset the enemies advantage in numbers.

Speaking of troop numbers, assuming I have some form of a garrisoned fleet I'd have to consider whether it would be possible to fight an enemy naval attack in order to defeat or to just delay the enemy long enough. The problem there being in the picture I see only 5 ports so I'm going to assume at the most a garrison fleet that I'd have would amount to no more than 5 ships. Not good if the enemy brings say 100 ships against me.

There is also the question of the time period and the type of soldiers that I can use. Is this the medieval era where I have levies and mounted knights fighting up close and personal with steel, or is this in the 1700-1800s where armies fought primarily with rifle and the use of cannons both normal sized for regular battles and humongous siege cannons are common.

In terms of defense, really it depends on the number of soldiers at my disposal versus the number of enemy soldiers. The makeup of the armies such as if I have levies and knights or men of the line with muskets and so on. What siege engines my enemy uses such as catapults, trebuchets, cannon, towers, ladders, or even sappers. The possibility of a naval assault or even a combined land and naval assault. Taking all of this into consideration it's obvious that this settlement could be defended but that really depends on how many men the enemy bring against me. Assuming this is the medieval period and the enemy has 5000 men assaulting this settlement and I have a garrison of 1000 men to defend it, I can say that I believe I'd win that battle. However if my garrison of 1000 men are facing a 20,000 man army, then I don't think I could hold the city and would most likely ask for terms of surrender.