Every rooftop has a low wall around it and has been capped off with a little cone of rubble.
In case anybody was wondering, this is most likely because Yemen has the same daft law as much of the Middle East: you only start paying property tax when your house is complete, so if you leave the "last floor" (roof) notionally incomplete (a row of bricks plus some construction detritus, as if you were still working on it), you never pay tax.
People have reported similar stories from Turkey and Greece, for what it's worth. No one seems to agree whether it's about tax stuff or leaving the option to extend dwellings by another floor.
In Yemen the practice is to have the top floor of a dwelling be the mufraj (room with views where men chew qat) which is what makes me skeptical about the tax idea there. The buildings certainly don't look unfinished from rooftop level.
The thing that makes me most skeptical, though, is just that it's such a wonderful, repeatable story.
In Brazil during the colonial times catholic churches would have to pay taxes once they were complete that is why they always have one bell tower and a second incomplete tower.
In case anybody was wondering, this is most likely because Yemen has the same daft law as much of the Middle East: you only start paying property tax when your house is complete, so if you leave the "last floor" (roof) notionally incomplete (a row of bricks plus some construction detritus, as if you were still working on it), you never pay tax.