Such bridge towers were common across Europe from medieval times, but many were destroyed due to urban expansion, diminishing defensive requirements and the increasing demands of traffic and trade.

Through the medieval era, the English Civil War, and the Chartist uprising, the bridge played a significant, if ineffectual, role in defending Monmouth.

[7] In 1988, work on flood defences revealed remains of the wooden bridge directly under the existing one,[8] and dendrochronological analysis indicated that its timber came from trees felled between 1123 and 1169.

[9][a] An early account in the Flores Historiarum by Roger of Wendover may indicate that the wooden bridge and the nearby Church of St Thomas the Martyr were damaged by fire in the Battle of Monmouth in 1233, fought between supporters of Henry III and the forces of Richard Marshal, Earl of Pembroke.

[15][b] The historian William Coxe incorrectly described the bridge as pre-dating the Norman Conquest and recorded that "it commanded the passage of the Monnow and was a barrier against the Welsh".

[17] In 1804, the Monmouth antiquarian Charles Heath wrote that the bridge's "foundation is so ancient that neither history or tradition afford any light respecting the date of its erection".

[19] The archaeologist Martin Cook notes the significance of the date 1270 as the start of a period that saw increased bridge-building, as a result of the rapid growth of international trade.

[20][c] The civil engineer Edwyn Jervoise suggested that the absence of an evidential record was due to the destruction of the archives of the Duke of Beaufort at Raglan Castle in the 17th century.

[16] The siting of the gatehouse mid-channel is relatively unusual; the archaeologist David Harrison notes the more common arrangement was for the gate to be situated on the roadway at one end of the bridge.

The prominent arched machicolations, defensive apertures through which stones or other material could be dropped on attackers, were added at an unknown date in the medieval period, possibly in the late 14th century.

The Victorian antiquarian Mary Ellen Bagnall-Oakeley, who wrote the first history of the bridge and gate in 1902, described the gatehouse as "a little fortress complete in itself, though of course, useless in time of war".

[33] Her account contended that the "tower was not in any way connected with the fortification walls of the town"[34] and that the gatehouse was erected solely "for the purpose of taking tolls".

[40] In the 16th century, the antiquarian John Leland described the bridge in one of his Itineraries: "From Monk's Gate the wall extends Westwards to the river Monnow.

"[41] A visual depiction of the bridge and gate is included in John Speed's work The Theatre of the Empire of Great Britain, published in 1611.

[44] By 1705, the bridge and gatehouse required maintenance: the original battlements were replaced with solid walls, and the building was refitted to form a two-storey dwelling house[45] with timber and lath extensions projecting over the river.

The Monmouthshire writer and artist Fred Hando records that, on a single Saturday in the early 18th century, "500 horses each carrying five bushels of corn entered by way of the Monnow gate".

[50] In 1804, Charles Heath recorded, "The interior has nothing worthy of attention and the only purpose to which it is employed is an occasional guardhouse, or powder room, for the military, when stationed at Monmouth.

[53] In a lengthy transaction, begun that year but not concluded until 1835,[54] ownership was formally transferred to the Duke of Beaufort as part of a property exchange.

In 1891, it was decorated with flags and lights to commemorate the coming of age of John Maclean Rolls, eldest son of Monmouthshire grandee Lord Llangattock.

[61] From 1889 to 1902, an extensive programme of conservation was carried out on the bridge and gate, directed by Monmouthshire County Council, which retained responsibility for maintenance.

Roof guttering and downpipes were added, badly eroded stone was replaced with squared blocks of Old Red Sandstone, and the cruciform arrowslit on the left-hand side of the west elevation of the gatehouse was restored to make it symmetrical.

[66] In 1963, Fred Hando, who recorded points of interest and history around Monmouthshire in a series of articles for the South Wales Argus between the 1920s and the 1960s, wrote a description of the gatehouse, referencing the small museum then located in the upstairs room.

[55] In the 20th century, the greater volume of traffic using the humpbacked bridge, which had poor visibility and narrow approach roads, led to a rise in accidents and an increase in bypass proposals.

The desire of local authorities to clear carriageways of obstructions to traffic led to the demolition of many similar bridge towers from as early as the 18th century.

[73][74] The closure to traffic also enabled a significant repair programme to be undertaken, in part funded by the Welsh Government and the European Union.

[79] The noted architectural watercolourist Samuel Prout painted the bridge in a study dated "before 1814", now held at the Yale Center for British Art in Connecticut.

[81] The watercolourist and etcher John Sell Cotman sketched the bridge in the early 19th century, his drawing showing the overhanging accommodation and guardhouse that were later removed.

[84] The artist of the American West Thomas Moran produced an undated pencil drawing of the bridge which is printed in his Field Sketches.

[85] A depiction of the bridge in stained glass by Charles Eamer Kempe can be seen in the Memorial of the Boer War window in St Mary's Priory Church in Monmouth.

[101] But urban expansion, the lessening of defensive requirements, and the substantial increases in traffic and trade from the 18th century onward led to the destruction of many of what was once a common bridge type.