Re: Datong Great Wall
Perhaps because a round tower is much stronger than a square or rectangular tower.
From the pictures is seems that the round towers are not connected the wall itself but is located at some distance. It that case it would make sense build a stronger round tower, since they to some extent might have to defend themselves. This might also explain why all the single towers in Liaoning are round.
A square tower connected to the wall was less vulnerable to enemy attack since it had the protection of the fortified wall, and it was easier to build and easier to utilise the internal space.