Posted at Oct 17/2007 04:26PM:
Karl Su: The utum (p. atam) was a defensive tower-house made out of stone and mud, commonly found in areas of the Hijaz like Yathrib/Medina and further south in Yemen. They had both defensive and residential functions. These fortified towers were representative of pre-Islamic tribal architecture and similar structures can be found in Saudi Arabia to this day. One theory is that they reflected the need for a safe tower in the times of civil strife when the Arabian peninsula had not yet been unified by Islam.

Paraphrased from: G. R. D. King Muqarnas, Vol. 8, K. A. C. Creswell and His Legacy. (1991), pp. 94-102.