If you only see one archaeological site in Libya, this is the one to choose. Regarded as the best Roman site in the Mediterranean, Leptis Magna's spectacular architecture and massive scale will impress even the most ruin-weary traveller.
The city was originally a Phoenician port, settled during the first millennium BC. Slaves, gold, ivory and precious metals brought it great wealth, which was supplemented by the rich agricultural land surrounding it. Roman legions ousted the Carthaginians following the third Punic War, after which the city flourished until the Vandals did their namesake thing in 455.
Roman rule briefly returned to Leptis in 533, and intensive repairs were carried out on the city, but local tribes revolted and eventually the area reverted to pastoral nomadism dominated by the Berbers. The Arab invasions of 644 swept away the last traces of Roman life from the region, and in the 11th century Leptis Magna was finally abandoned to the encroaching sand dunes.
It wasn't until the 20th century that excavation began in earnest, and, much to archaeologists' delight, the sands had preserved the ruins remarkably well. There's an excellent, large museum next to the main entrance to the site, but the real treasures wait out in the site itself.
The first thing you'll encounter is the Severan Arch, which was erected in honor of Emperor Septimus Severus' visit to his hometown in 203 AD. Not far off are the marble and granite panelled Hadrianic Baths, the largest outside Rome. Keep exploring and you'll come across the partially covered nymphaeum , a shrine dedicated to the worship of nymphs; a pair of massive forums, similar in design and grandiosity to the imperial forum in Rome; the extraordinarily detailed basilica and theatre; and, if you continue west along the seashore about 700m (2100ft), the circus and amphitheatre, where chariot races and similar spectacles were held for the locals' amusement.
Note that there are separate entry fees for the Leptis museum and amphitheatre (equal to the entry fee) and it is compulsory to have a guide to visit Leptis Magna.