The castles float between the Western Cloudwall and the Eastern Cloudwall and are full of semblances. The Sky Raiders enter the castles and take inanimate semblances from the castles. They then sell or barter the "stolen" semblances to persons from throughout The Outskirts.
"The life expectancy for a new slave among them is around two weeks" (1:60). New slaves are used as scouts to scout out the castles. If the castle is dangerous, the raiders move on to another castle. If the castle is safe, other raiders will land and start taking away goods. The odds of surviving are low, with only about one in twenty new scouts completing their required 50 missions before moving on to less dangerous work.