The essential function of all tall towers is to enable broadcasters to transmit line-of-sight signals to their customers.

"Broadcasting is a big stick industry, where one tower can cover a whole market," according to Kelly Williams, senior director of Engineering for the National Association of Broadcasters. "You put up a tower to facilitate broadcasting wirelessly."

Towers host a wide variety of antennae, which transmit over the air (analog) television signals, AM and FM radio, telco long lines (which transmit data), pager signals and emergency service channels (including two-way radio).

But tower owners have long realized that their structures have tourism potential. In addition to running observation decks, almost all major towers have restaurants, most of which revolve slowly to offer patrons effortless panoramic views.

Jack Robinson, chief operating officer of CN Tower in Toronto, the tallest freestanding tower in the Western Hemisphere, outlines owners' changing business model:

"We started off in 1976 as a broadcast tower, that was our core business. We have since morphed into a fabulous entertainment complex, with the observation gallery, three restaurants and Edgewalk, which allows thrill seekers to walk along the edge while attached to a harness. Our 360 Restaurant is probably the number one restaurant for marriage proposals in Canada."

CN Tower was built by Canadian National Railway, which was government owned. Since 1995 the structure has been owned by Canada Lands Co., a federal crown corporation operating under the Ministry of Public Works. An expansion in 1998 added a 3D cinema and shopping arcade at the base. The tower is operated for profit, but does not break out figures, and consistently draws 1.5 million customers a year.

"Revenues are up four or five percent every year. We've gone from pretty much a broadcast tower to a vertical theme park," says Robinson.

The U.S. does not make the cut among the 10tallest towers featured in the accompanying slide show. The tallest freestanding tower in the U.S. is the Stratosphere, a 350-meter-tall tower attached to a casino in Las Vegas. It is the 17th-tallest freestanding tower in the world.