I'm about 90% done with this project, which is good because I want it finished by the end of the month. Here's what's completed and what is lacking:
I need to finish the farm and port section. I think there will only be a single ramshackle building at the port and the farm will not be too large either. Now if you look at it, the island lacks vegetation, which can be remedied by adding static grass and the farm will have some green pastures as well.
On the other hand, I've completed the cliff with its removable top (it can be used as a walled garden or somesuch in other games), and painted the internal space as well - it has a nice consistent look. I just used bright brown (cheap acrylic paint bought at a supermarket and titled 'Martian Brown') and sand for most of the groundwork, and grey, sand and white for the cliff and buildings.
And here's the small island with the rotunda complete. I cut out the walls from a single piece of pink foam and marked them with a pen. The rotunda and fortress door are removable single pieces so the attackers can bash them down.
I had to think about what sort of cannon to place inside, finally I went with large naval guns that can strike fear in any approaching ship; also I though about building a stone bridge but then decided that a large wooden one will do. Those thick planks can support large artillery pieces, and if the wooden bridge is broken down by enemy gunfire, it's easier to rebuild; I think the water between the two islands is shallow enough to either wade or easily swim through anyway.