The rule says that construction of buildings must go on empty sky island tiles in your charter. Later in the Chronicle, the constructions rules indicate that buildings may be built on empty OR ON TOP OF buildings in your charter (provided there are no tokens on it). Does this include sky island tiles? Or is it intended that sky island tiles can only be built on once?
We didn't read the FAQ on that and didn't play that way, so our Sky Islands were a no go zone, but it seemed to have a very small practical effect on the game, if any.