Since you are considering things like "The Great Tower", I will make suggestions heavily utilizing magic in the construction.
First you want to find someplace that already has a Magic Aura as your base if possible, since this will make life easier in the long run. A small island, to tiny for normal use, would be perfect as the Aura anchor. This also ensures you are not trying to build your island in extremely deep water (the Med averages 1,500m in depth for example).
Using the level 30 to form the outer wall in a open ended cylinder, modified to create a four sea gates (+1 Mag). Use another version of that spell to create a open ended roughly rectangular prism in the middle around your anchor island. You can make the walls slightly higher and dimple in the walls in line with the sea gates.
If you want a truly epic tower, consider using a solid stone cylinder as the base. If you are willing to use workmen and/or magic to shape it, then a higher level version can create a huge cylinder which since it can get by without the added magnitudes for Complexity. The level 35 will give you 240 paces of height and over 81 paces of diameter.
For back-fill I would recommend clay with two paces of top soil, since this will provide more support than straight dirt, while being waterproof and allowing things like underground cisterns.
If you pick a base location not overly deep (that anchor rock), you will end up with a fortified ring some ~3 miles in diameter, with four sheltered harbors accessible by sea gates, a square island inside the outer wall (even greater in height and connecting to it at its four corners), and a stone cylinder 240 paces high and over 81 paces in diameter. It would have an ablation layer and back fill of clay (with 2 pace topsoil, level 30). That should be more than enough space for your needs.