Lighthouse of Alexandria
Egypt

Why this ancient Seven Wonders structure was special
The Lighthouse of Alexandria is one of the greatest architectural feats of antiquity. It was at least as tall as a 40 story modern building and stood for over 16 centuries.

Interesting tidbits about 
the Lighthouse of Alexandria
Unlike the other six ancient Seven Wonders of the World, the Lighthouse of Alexandria had practical purposes. It helped seafaring ships find the harbor and then safely enter it (treacherous shoals existed just outside the harbor). The lighthouse also served as a military lookout for approaching enemy ships. It was also a tourist destination (with two observation platforms).
It was obviously solidly constructed. It survived for some 1600 years despite being assaulted by fierce winter sea storms and many earthquakes. 
The Lighthouse of Alexandria employed two types of beacons placed near the building's summit. At night, a bonfire generated the light. During the day, a mirror created it by reflecting and directing sun rays. The mirror was probably a large, round, concave disc made of polished metal.
A fanciful account would have us believe that the mirror was used to burn approaching enemy ships by focusing the sun rays on them. 
The Lighthouse of Alexandria was conceived in the early 3rd century BC by the ruler Ptolemy I, an ex-general of Alexander the Great, the conqueror who founded and gave his name to Alexandria. Ptolemy II (the son of Ptolemy I) finished constructing the Lighthouse of Alexandria about 25 years later.
The Lighthouse of Alexandria had three basic structural elements. A rectangular base, an octagonal midsection, and a cylindrical upper section which housed the beacon (see above picture).
Estimates for the combined height of the three sections range from 100 to 180 meters (330 to 600 feet). A consensus of reliable sources sets the figure at about 120 meters (400 feet).
Except for the great pyramids of Egypt, the Lighthouse of Alexandria was the tallest structure in the world until the Eiffel Tower was erected in 1889.
During the 1300's, the gradual disintegration of the Lighthouse of Alexandria accelerated. The chief culprits were human neglect and a series of unusually severe earthquakes. By 1400 AD, the Lighthouse of Alexandria was in ruins.
What happened to the fallen building blocks of the Lighthouse of Alexandria? Some were used by Sultan Qaitbay in the mid 1400s to build a fortress on (or near) the lighthouse's site. Today, this seaside fort is the focal point of Alexandria's harbor and is as sturdy as ever.
In 1994, a scuba-diving archeological expedition explored and mapped the harbor's seafloor near the lighthouse's site. They located over 2,000 submerged large building blocks, which they say were part of the lighthouse. Some critics believe that the stones have nothing to do with the lighthouse because many of the sunken objects came from even earlier ancient Egyptian buildings. The archeologists counter argue that the ancient Egyptians commonly recycled material from old structures - and the builders of the Lighthouse of Alexandria were no exception.
The structure is also well known as the "Lighthouse of Pharos." It was named for the small island Pharos in Alexandria harbor upon which the lighthouse was built. A causeway connects the island with the mainland. 
The Lighthouse of Pharos became so renowned that the name "pharos" is the root of the word "lighthouse" in several languages (for instance, "phare" in French).