Limes Germanicus

The first emperor who began to build fortifications along the border was Augustus, shortly after the devastating Roman defeat at the Battle of the Teutoburg Forest in 9 AD. Originally there were numerous Limes walls, which were then connected to form the Upper Germanic Limes along the Rhine and the Rhaetian Limes along the Danube. Later these two walls were linked to form a common borderline.

From the death of Augustus (14 AD) until after 70 AD, Rome accepted as her Germanic frontier the water-boundary of the Rhine and upper Danube. Beyond these rivers she held only the fertile plain of Frankfurt, opposite the Roman border fortress of Moguntiacum (Mainz), the southernmost slopes of the Black Forest and a few scattered bridge-heads. The northern section of this frontier, where the Rhine is deep and broad, remained the Roman boundary until the empire fell. The southern part was different. The upper Rhine and upper Danube are easily crossed. The frontier which they form is inconveniently long, enclosing an acute-angled wedge of foreign territory between the modern Baden and Württemberg. The Germanic populations of these lands seem in Roman times to have been scant, and Roman subjects from the modern Alsace-Lorraine had drifted across the river eastwards. The motives alike of geographical convenience and of the advantages to be gained by recognising these movements of Roman subjects combined to urge a forward policy at Rome, and when the vigorous Vespasian had succeeded Nero, a series of advances began which gradually closed up the acute angle, or at least rendered it obtuse.

Roman limes and modern boundaries.

The first advance came about 74 AD, when what is now Baden was invaded and partly annexed and a road carried from the Roman base on the upper Rhine, Straßburg, to the Danube just above Ulm. The point of the angle was broken off.

The second advance was made by Domitian about 83 AD. He pushed out from Moguntiacum, extended the Roman territory east of it and enclosed the whole within a systematically delimited and defended frontier with numerous blockhouses along it and larger forts in the rear. Among the blockhouses was one which by various enlargements and refoundations grew into the well-known Saalburg fort on the Taunus near Bad Homburg. This advance necessitated a third movement, the construction of a frontier connecting the annexations of 74 and 83 AD. We know the line of this frontier which ran from the Main across the upland Odenwald to the upper waters of the Neckar and was defended by a chain of forts. We do not, however, know its date, save that, if not Domitian's work, it was carried out soon after his death, and the whole frontier thus constituted was reorganised, probably by Hadrian, with a continuous wooden palisade reaching from Rhine to Danube.

The tower "WP 14/55" at the Upper German-Raetian border wall

The angle between the rivers was now almost full. But there remained further advance and further fortification. Either Hadrian or, more probably, his successor Antoninus Pius pushed out from the Odenwald and the Danube, and marked out a new frontier roughly parallel to, but in advance of these two lines, though sometimes, as on the Taunus, coinciding with the older line. This is the frontier which is now visible and visited by the curious. It consists, as we see it today, of two distinct frontier works, one, known as the Pfahlgraben, is a palisade of stakes with a ditch and earthen mound behind it, best seen in the neighbourhood of the Saalburg but once extending from the Rhine southwards into southern Germany. The other, which begins where the earthwork stops, is a wall, though not a very formidable wall, of stone, the Teufelsmauer; it runs roughly east and west parallel to the Danube, which it finally joins at Heinheim near Regensburg. The southern part of the Pfahlgraben is remarkably straight; for over 50 km (31 mi) it points almost absolutely true for Polaris.

This frontier remained for about 100 years, and no doubt in that long period much was done to it to which precise dates are difficult to fix. It cannot even be absolutely certain when the frontier laid out by Pius was equipped with the manpitts and other special fortifications. But we know that the pressure of the barbarians began to be felt seriously in the later part of the second century, and after long struggles the whole or almost the whole district east of the Rhine and north of the Danube was lost, seemingly all within one short period, about 250.

Germanic invasions in the late third century led to the abandonment of the so-called "Upper Raetian Limes" in favour of a Roman defence line along the rivers Rhine, Iller and Danube (Donau-Iller-Rhine-Limes). Support was provided to some degree by fast river boats, the navis lusoria being the standard type, that could reach outposts or points of crisis quickly. Watch towers were in sight contact and heavily fortified castra placed at important passes (e.g. Castrum Rauracense instead of the previously unwalled Augusta Raurica near to Basel) and in the hinterland of the frontier (e.g. Vindonissa in today's Switzerland).

• Tabula Peutingeriana

• This article incorporates text from a publication now in the public domain: Chisholm, Hugh, ed. (1911). "Limes Germanicus". Encyclopædia Britannica. Vol. 16 (11th ed.). Cambridge University Press. pp. 695–696.
• A good English account can be found in H. F. Pelham's essay in Trans. of the Royal Hist. Soc. vol. 20, reprinted in his Collected Papers, pp. 178–211 (Oxford, 1910), where the German authorities are fully cited.
• D.J. Woolliscroft, Roman Military Signalling. Stroud and Charleston: Tempus Publishing, 2001. p. 191. ISBN 0-7524-1938-2. A study mainly of intervisibility along the Rhine and British limites.