Islands are geographical features distinguished by isolation from main continents. Island fortresses are characterized by inaccessibility to most caravans and invaders. Only civilizations located on the island, and your parent dwarf civilizations, will be able to make it to an island fortress. The only exception to this is if the island is in close enough proximity to land, and in a Cold or Freezing biome, so that the ocean is able to freeze and caravans are able to cross.
Advantages of islands
- The lack of goblin and kobold civilization access makes your fortress a peaceful location to learn how to manage the game's less military aspects.
- No military interruptions makes it easy to build megaprojects.
- Easy access to a large body of water for underwater megaprojects.
Disadvantages of islands
- The lack of invasions can get dull. An idyllic island paradise is all very well and good, but without constant slaughter your dwarves will have nothing to engrave about except the vermin they like least.
- No non-dwarven caravans on most smaller islands. You won't be able to collect bins of crappy cloth nearly as effectively, and won't be able to collect large bronze helmets at all.
- Megabeasts, titans, and forgotten beasts can still arrive on an island. This can be exceedingly fun if a lack of other invaders led you to get complacent.
- No missions. As they can only be done if your dwarves can physically reach the assigned location, you won't be able to recover artifacts or raid sites if you're confined to a small patch of land surrounded by ocean water.v0.44.01
- Depending on the island's size and biomes, it may be unlikely (if not outright impossible) for off-site holdings to form around an island fortress, and because of the aforementioned lack of missions, you won't be able to conquer sites. This means you won't be able to upgrade your fortress' nobility past it being a barony.v0.44.11 Depending on your disposition towards nobles, however, this may not be too much of a problem.
Reaching islands can be a difficult challenge. One way would be to swim it, and brave the hazards of the ocean and climate. Another way would be to cross it through the caverns but that may present another set of obstructions and dangers. You may also try to establish your own camps to build a bridge across the ocean tile, making the return trip much easier. The easiest way to reach an island however is with a giant variant of any flying animal as your pet which can then be ridden across any body of water.