Castellum

Built in 333 CE by order of Constantine, the Roman castellum at Traiectum ad Mosam was a relatively late addition to the Roman landscape. Its primary function was to secure the bridge rather than operate as a permanent military outpost. It remained in use in that capacity well into the 10th Century. The Romans understood that the Maas river crossing would prove to be of strategic importance from the Roman era, and it continued to be significant even into World War II.

Scale model in the Centre Céramique at Maastricht. Showing the late-Roman period castellum of Traiectum ad Mosam, built in 313 CE.
Scale model in the Centre Céramique at Maastricht. Showing the late-Roman period castellum of Traiectum ad Mosam, built in 313 CE. (Image credit: Kleon3, Source: Vici.org, CC BY-SA 4.0)

The rectangular Roman castellum was made of locally-quarried sandstone brick, cut to the thickness of 230 cm for the substratum and 140 cm above ground, and extended from eight to ten meters in height. It was internally framed by ten towers nine meters in diameter each with a height of fifteen to twenty meters. Two of the towers also functioned as lighthouses for the unpredictable Maas water levels to prevent boats from banking along its edge during rough weather. Four of the towers have been excavated. Upwards of four meters height of scattered remains remain dotted along its original wall outline in what is now the Stokstraat area. Indeed, this neighborhood’s street plan still retains the shape of the walls as the medieval city was laid out around these remains.

Excavated Roman castellum tower on Houtmaas Street, Maastricht.
Excavated Roman castellum tower on Houtmaas Street, Maastricht.
(Image credit: Rijksdienst voor het Cultureel Erfgoed; CC BY-SA 3.0)

Solid red lines denote excavated archaeological remains of the Roman castellum on the Western bank of the Maas River at Traiectum ad Mosam.

Solid red lines denote excavated archaeological remains of the Roman castellum on the Western bank of the Maas River at Traiectum ad Mosam. (Image credit: Jona Lendering, Source: Livius.org, CC0 1.0 Universal

The castellum was surrounded on three sides by a nine-meter wide defensive canal, with the South end filled by the Jeker River, and the North and West as dry defensive canals at a depth of nearly four meters. The west wall had no need for a canal as it was fronted by the Maas River. A gate, complete with a gatehouse, was positioned at the East entrance and opened out to the entry of the bridge making it more defensible. And a horreum (granary) was added to the castellum during a period of refortification in the Fourth Century. 

Roman blocks of stone from the Castellum wall. Now incorporated into the corner of the 11th Century CE church, Onze Lieve Vrouwebasiliek.
Roman blocks of stone from the Castellum wall. Now incorporated into the corner of the 11th Century CE church, Onze Lieve Vrouwebasiliek. (Image credit: Kleon3; Public Domain)

It is probable that a smaller castellum was built on the opposite bank of the bridge, based on typical Roman engineering schemes. The location of such a structure would have been in what is now the Wyck neighborhood of Maastricht, but there have not been excavations on this bank to prove this theory. However, it is likely that a secondary castellum would have been added to maximize the defensibility of the bridge from both directions of the Via Belgica.