I said that ships couldn't fit in, like merchant vessels you would use to transport across the sea. Rowboats would fit and could be used for transport. The reason for not building any sort of path to the outside is the precise reason it is giving our characters problems in the first place: it is more easily defensible. In order to get in, an enemy would have to either swim, or use a rowboat, and a rowboat would make it obvious that they were being attacked, not to mention that it would be an easy target for archers. It does also mean they wouldn't be able to get out as easily, but in the case of an attack, they would be wanting to defend anyway, as that is where all their valuables are.