Short answers, pulled from the story.
The Roman limes stretched for more than 10,000 kilometers. It ran from the Atlantic coast of northern Britain through Europe to the Black Sea, and from there to the Red Sea and across North Africa to the Atlantic coast.
Before it denoted a military frontier, limes carried several meanings in Latin: a path or balk delimiting fields, a boundary line or marker, any road or path, any channel such as a stream channel, or any distinction or difference. Its first recorded use to mean "land border" appears in Tacitus in 98 AD.
The limitanei were border soldiers not expected to win large-scale wars. Their role was to deter small-to-medium-sized raiding parties and hold the frontier until heavier forces could respond.
The Fossatum Africae was a linear defensive structure extending over at least 750 kilometers in northern Africa. It consisted of a ditch with earth embankments on either side, sometimes supplemented by dry stone walls, with a standard width of 3-6 meters and a depth of 2-3 meters as found near Gemellae. It was accompanied by watchtowers and forts built within sight of one another.
The Limes Arabicus ran from the Gulf of Aqaba through Palestine to Syria, covering about 1,500 kilometers at its greatest extent, and protected the province of Arabia Petraea from desert raiders for over five centuries. Trajan ordered the construction of the Via Traiana Nova between 111 and 114 as its primary military road, linking Aelana on the Red Sea with the fortress of Bostra, 267 Roman miles away. By the first half of the 6th century, the Roman troops there were replaced by Arab foederati, particularly the Ghassanids.
The Upper Germanic-Rhaetian Limes was a frontier barrier built from about 162-63 AD guarding the provinces of Germania Superior and Rhaetia. It ran approximately 550 kilometers from Rheinbrohl in northern Rhineland-Palatinate to Hienheim on the Danube, featuring watchtowers, palisades, ditches, and earthworks, with one section including a solid stone wall.