I'm a DM for two different on-going campaigns in a homebrew world. This summer, we've started up a secondary campaign that is nautically themed. I want the players to encounter a floating island with a castle on it. The island would be able to move; in fact, it'd be steerable and maneuverable with the right team of wizards and such.
How do you guys suggest I go about designing this castle? What mechanics are making it work? (I imagine magic of some sort, but I'm not entirely sure what.)
Any number of ways. Ask yourself a couple of questions, first:
What flavor/theme do you want the island to have?
There's a few basic routes that directly impact how the island works and it's method of propulsion.
Magical Theme: Island uses lots of magic. However it's done, it's magical. So the island is moved by enchanted breezes blowing sales, oars that row themselves, Decanters of Endless Water on Gyser mode, whatever.
Science Theme: Island uses lots of tech. However it's done, it's science-y. Variation on Magical theme, really, but more steampunk-ish. Boilers running water wheels, nuclear powered turbines, whatever.
Animals Theme. The island is "nature friendly" to an extent - much of what they do revolves around training and making use of some form of animal life (variation: Plant life that can be made to work in a similar manner). However they do it, it involves lots of critters (or a small number of really big critters) dragging things around.
Labor Theme. The island is run by mundane hard work. Perhaps half the population mans oars. Perhaps they've got some REALLY big sails and anchors. Perhaps someone has a really good understanding of oceanic currents, and they use controlled drops of water-drag anchors at different depths to steer (basically, a chain with a big object at the end which is designed to produce drag at the end, but not along the chain - you drop the end into a current, and the object is dragged along with the current... applying a bit of force to the island, and moving it. When you want to go a different direction, you find a different current; if you want to stop, you drop it all the way to the ocean floor - you can go almost anywhere... eventually... but the schedule and route will always be funny). They may be slaves, they may be paid, but by whatever means, it's done by people.
What role do you want the floating island to play in the game?
Do you want it to simply be a pretty piece of background? Do you want it to provide a single adventure hook? Multiple adventure hooks? This doesn't directly impact things... however, it does change flavors around.
If you want it to be a pretty piece of background only, then you simply make everything as it appears to be - the place is straightforward and honest.
If you want it to provide a single adventure hook, then it seems to be one of the above on the surface, but is secretly a different version - perhaps the animals towing the place are actually intelligent creatures, compelled to labor against their will. Perhaps those magical, sourceless oars / winds are actually powered by sacrificing the soul of someone once a month. Perhaps all the technology is only window-dressing, to distract from the actual source, which turns out to be mundane magic.
If you want it to provide multiple adventure hooks, then you arrange for everything to seem mostly honest, but just be slightly off.
