Maybe everybody else in the world knew this already, but I didn't. The architect of the WTC towers designed them specifically as references to Mecca:

Yamasaki received the World Trade Center commission the year after the Dhahran Airport was completed. Yamasaki described its plaza as "a mecca, a great relief from the narrow streets and sidewalks of the surrounding Wall Street area." True to his word, Yamasaki replicated the plan of Mecca's courtyard by creating a vast delineated square, isolated from the city's bustle by low colonnaded structures and capped by two enormous, perfectly square towers--minarets, really. Yamasaki's courtyard mimicked Mecca's assemblage of holy sites--the Qa'ba (a cube) containing the sacred stone, what some believe is the burial site of Hagar and Ishmael, and the holy spring--by including several sculptural features, including a fountain, and he anchored the composition in a radial circular pattern, similar to Mecca's.

View of a World Trade Center TowerView of a World Trade Center towerAt the base of the towers, Yamasaki used implied pointed arches--derived from the characteristically pointed arches of Islam--as a transition between the wide column spacing below and the dense structural mesh above. (Europe imported pointed arches from Islam during the Middle Ages, and so non-Muslims have come to think of them as innovations of the Gothic period.) Above soared the pure geometry of the towers, swathed in a shimmering skin, which doubled as a structural web--a giant truss. Here Yamasaki was following the Islamic tradition of wrapping a powerful geometric form in a dense filigree, as in the inlaid marble pattern work of the Taj Mahal or the ornate carvings of the courtyard and domes of the Alhambra.

The shimmering filigree is the mark of the holy. According to Oleg Grabar, the great American scholar of Islamic art and architecture, the dense filigree of complex geometries alludes to a higher spiritual reality in Islam, and the shimmering quality of Islamic patterning relates to the veil that wraps the Qa'ba at Mecca. After the attack, Grabar spoke of how these towers related to the architecture of Islam, where "the entire surface is meaningful" and "every part is both construction and ornament." A number of designers from the Middle East agreed, describing the entire façade as a giant "mashrabiya," the tracery that fills the windows of mosques.

Link via Capitalism's Mecca, original article at Bin Laden's special complaint with the World Trade Center. - By Laurie Kerr - Slate Magazine.