The Limes Tripolitanus was a frontier zone of defence of the Roman Empire, built in the south of what is now Tunisia and the northwest of Libya. It was primarily intended as a protection for the tripolitanian cities of Leptis Magna, Sabratha and Oea in Roman Libya.
The Limes Tripolitanus was built after Augustus. It was related mainly to the Garamantes menace. Septimius Flaccus in 50 AD did a military expedition that reached the actual Fezzan  and further south.
The Romans did not conquer the Garamantes so much as they seduced them with the benefits of trade and discouraged them with the threat of war. The last Garamantes foray to the coast was in AD 69, when they joined with the people of Oea (modern Tripoli) in battle against Leptis Magna.
The Romans, in order to defend the main Roman cities of Tripolitania (Oea, Sabratha and Leptis Magna), intervened and marched south. According to Edward Bovill, author of the book "The Golden Trade of the Moors", this campaign marked the Romans’ first use of camels in the Sahara, which convinced the Garamantes that their advantage in desert warfare no longer held.
After that the Garamantes started to become a client state of the Roman Empire, but nomads always endangered the fertile area of coastal Tripolitania. Because of this Romans created the Limes Tripolitanus 
The first fort on the limes was built at Thiges, to protect from nomad attacks in 75 AD. The limes was expanded under emperors Hadrian and Septimius Severus, in particular under the legatus Quintus Anicius Faustus in 197-201 AD.
Indeed, Anicius Faustus was appointed legatus of the Legio III Augusta and built several defensive forts of the Limes Tripolitanus in Tripolitania, among which Garbia  and Golaia (actual Bu Ngem) in order to protect the province from the raids of nomadic tribes. He fulfilled his task quickly and successfully.
As a consequence the Roman city of Gaerisa (actual Ghirza), situated away from the coast and south of Leptis Magna, developed quickly in a rich agricultural area Ghirza became a "boom town" after 200 CE, when the Roman emperor Septimius Severus (born in Leptis Magna) had organized the Limes Tripolitanus.
Former soldiers were settled in this area, and the arid land was developed. Dams and cisterns were built in the Wadi Ghirza (then not dry like today) to regulate the flash floods. These structures are still visible: there it is among the ruins of Gaerisa a temple, which may have been dedicated to the Berber semi-god "Gurzil", and the name of the town itself may even be related to his name. The farmers produced cereals, figs, vines, olives, pulses, almonds, dates, and perhaps melons. Ghirza consisted of some forty buildings, including six fortified farms (Centenaria). Two of them were really large. It was abandoned in the Middle Ages.
With Diocletian the limes was partially abandoned and the defence of the area was done even by the Limitanei, local soldier-farmers. The Limes survived as an effective protection until Byzantine times (Emperor Justinian restructured the Limes in 533 AD).
Nomad warriors of the Banu Hillal tribe captured the centenaria/castra of the Limes in the 11th century and the agricultural production fell to nearly nothing in a few decades: even Leptis Magna and Sabratha were abandoned and only Oea survived, which was from now on known as Tripoli.
In Libya today, very substantial remains survive, e.g., the limes castles at Abu Nujaym (ancient Golaia) and Al Qaryah al Gharbīyah, the frontier village Gaerisa, and about 2,000 fortified farms (Centenaria) like Qaryat.
Some Forts (castrum)
- Jona Lendering (December 13, 2008) [2006, revised]. "Limes Tripolitanus". Livius. Retrieved October 3, 2011.
- Jona Lendering (December 14, 2008) [2006, revised]. "Wadi Buzra / Suq al-Awty". Livius. Retrieved October 3, 2011.
- Jona Lendering; Marco Prins. "Photos from Libya". Livius. Archived from the original on April 5, 2008. Retrieved October 3, 2011.
- "Detailed map showing the Limes Tripolitanus at Tunisia-Libya border". Georgetown University. Retrieved October 3, 2011.
- Bacchielli,L. La Tripolitania in "Storia Einaudi dei Greci e dei Romani" (Geografia del mondo tardo-antico). Einaudi Ed. Milano, 2008.
- Graeme Barker e.a., Farming the desert. The UNESCO Libyan Valleys Archaeological Survey (1996 Paris and Tripoli)
- Margot Klee, Grenzen des Imperiums. Leben am römischen Limes (2006 Stuttgart)
- Jona Lendering, 'Sherds from the Desert. The Bu Njem Ostraca' in: Ancient Warfare 1/2 (2007)
- David Mattingly, Roman Tripolitania (1995 London)
- Erwin Ruprechtsberger, Die römische Limeszone in Tripolitanien und der Kyrenaika, Tunesien - Libyen (1993 Aalen)
- Livius.Org Golaia (Bu Njem)
- Septimius Flaccus
- Map of Limes tripolitanus
- Gheriat el-Garbia
- J.S. Wacher, The Roman world, Volume 1, Taylor & Francis, 2002, ISBN 0-415-26315-8, pp. 252-3
- Jona Lendering (May 23, 2009) [2006, revised]. "Ghirza: Town (Gh127)". Livius. Retrieved October 3, 2011.
- al_allgi (November 5, 2008). "Tarhuna: A map of the cultivated libyan lands in ancient times". Flickr. Retrieved October 3, 2011.
- "Ghirza National monuments". LookLex. Retrieved October 3, 2011.
- René Basset (1910). "Recherches Sur La Religion Des Berberes" [Research on Berber Religion]. Revue de L’Histoire des Religions. Retrieved October 3, 2011. (French)
- Bacchielli, L. La Tripolitania, in "Storia Einaudi dei Greci e dei Romani", Geografia del mondo tardo-antico, vol.20
- Jona Lendering (April 14, 2009) [2006, revised]. "Gheriat esh-Shergia". Livius. Retrieved October 3, 2011.