Vera Parvassi's Flying Tower

From the introduction:

Vera Parvassi's flying tower is a legend around this part of the world - generations of its denizens tell stories about the building in the shape of an hourglass, which roams between the clouds, and of the enigmatic witch that observes all from above. This time, though, something seems different: the witch's tovwer has frozen in mid-air for three whole days - and the rock beneath it has begun to crumble. The locals are worried that the tower has started to fall apart, and that the consequences could be devastating...