People have been building towers for as long as there have been cities. From the watchtowers and temple spires of ancient cities to the skyscrapers and radio towers that form the most modern skylines, towers represent the ongoing evolution of architectural and engineering techniques. The structures offer some major advantages but also pose serious challenges to designers and builders.
One of the biggest benefits of towers is the space advantage they provide. Storage towers like silos make it easy to store items above the ground, keeping them safe from water and animals without occupying a large area. Tall buildings like skyscrapers use multiple floors to increase total floorspace without increasing the size of the building's footprint, making the design ideal for a congested city where real estate is at a premium.
Towers also provide long-range views for people at or near the top level. In modern office buildings and residential towers, this means attractive vistas that may rise over a congested city or afford views of nearby landmarks and natural formations.
Other towers offer views with a more practical function. Watchtowers in forests give observers a clear view of where fires start, making them easier to contain. Lookout towers on military sites or in ancient cities provide a view of oncoming enemies, making the site easier to defend.
Towers also provide symbolic benefits that other types of buildings lack. Besides proclaiming the architectural abilities of a given nation or culture, people have often built towers to commemorate important events or honor important individuals. For example, the Eiffel Tower was built for the Paris Exposition of 1889 to mark the 100th anniversary of the French Revolution. Today it is major symbol of the city of Paris. Other symbolic and famous towers include the Petronas Twin Towers in Malaysia, the Chrysler Building and Empire State Building in New York City, the leaning tower of Pisa in Italy and the CN Tower in Toronto.
Towers provide excellent sites for installing broadcasting equipment. By placing an antenna at the highest point in a city, broadcasters can send a powerful signal that will reach television viewers, radio listeners or cell phone users for many miles. Placing broadcasting antennas on towers (rather than natural formations like mountains) also makes them more accessible for service and maintenance.
Safety is one of the major disadvantages of a tower. Defective towers may pose a risk of falling and causing severe damage on the ground. In addition, towers present a special challenge for evacuations, since those on higher floors need special means of escape in the event of a fire on a lower floor.
Architects and engineers must be attentive to safety when designing towers. Every tall structure is subject to powerful winds and low temperatures that occur at high altitudes, and towers must be able to withstand such forces while also handling the weight of the structure itself and external force from earthquakes.
Finally, safety is a major concern during the construction of a tower. Workers must use harnesses, safety nets and scaffolding to build and repair towers to prevent dangerous falls.
Towers are often more expensive than lower structures. Besides costing more to construct, towers require more money and work hours to clean, repair and renovate. The tallest towers make use of new technologies like special lightweight building materials, shock absorbers and mass dampers to prevent excessive swaying. All of these systems add to the cost of a tower, making it economically feasible only in certain cases.