Despite the issues with the foundations, the air is not such a bad location for a castle. I can think of worse, such as at the bottom of the sea, or in a stomach if one large enough could be found.
The advantage of being in the air, should the castle somehow contrive to remain their, is obvious; death can be rained from a great height without much worry of return fire, and the knights can wear aeroplane goggles and make engine noises. There are one or two drawbacks such as not having any easy way to resupply when besieged; however, if the castle can land, doing so on the besiegers should kill two birds with one pile of stones.
Sadly, castles in the air never became more than an idiom; such a tragic fate is not uncommon, but the loss of knights going ‘brrrrrmmmmwhhooosh’ is surely one of the greatest tragedies to have ever occurred.
Severely odd 