The discovery of Building J predates the excavations that revealed the existence of the 4th century castellum. As early as 1902-1903, city architect W. Sprenger (1914) uncovered the western facade masonry of a large Late Roman building during the construction of the new Parochial School at the northeastern region of the Church of Our Lady (OLV-church). Later, in 1918 and then 1924-1926, Goossens (1926) found further remains of the entire southern side and fragments of the eastern section of the building in the playground south of the school and in the buildings across the Stokstraat. At the time, Goossens believed it to be a pre-medieval predecessor of the OLV-church due to the presence of sequentially-spaced spolia within the interior and along the building’s western facade. However, the identification of the building as Roman didn’t occur until excavations conducted by Boagers in 1964 and 1966 that the entire floor was revealed (Boagers, 1964; 1966) followed by a redevelopment of the Stokstraat quarter in 1968 in which the a large section of the northern wall building was discovered (Panhuysen, 1996), thus nearly completing the entire picture of Building J.
Building J was located about 25 meters south of the inner section of the roman road and faced the western gate entrance. Externally, the building measures at 16.13 by 32.40 meters and internally at 14.30 by 30.90 meters. The walls are technically constructed in a similar way to the castellum walls. The masonry follows an opus caementicium technique with a facing of small, primarily coal sandstone blocks inserted occasionally with red bands of tiles and roof shingles. Also like the fortification walls, there is a stepped offset system in the lower section that marked the transition between foundation wall and substructure wall. On the external face, there was a single stepped offset of 28 cm deep while the internal face included multiple stepped offsets.
The interior is equally divided into three naves by two rows of reused spolia stones, each nave is a bit larger than five meters wide. It was suggested by Goossens (1926) that these stone dividers were used as support bases for the wooden constructions for the building’s elevated floor. While the exact location of the entrance is not determined, it is shown that the building had a large entrance hall facing the west. While Building J’s technical analysis and structural comparisons aligned it with a military purpose, its exact function was speculated on until the 1980s when it was suggested as a double horreum, at least for its initial phases of construction and usage during the 4th century occupation (Panhuysen, 1996).