The Roman West Gate of the Colonia (vanished city gate at the Apostelnkloster)

The gate belonged to the western wall of the Roman town and lay between the south-west gate and the northern honorific gate at the western end of today's Breite Straße. From here the Roman state road ran straight westward towards Boulogne-sur-Mer; it is known today as the Via Belgica and roughly follows the course of the Aachener Straße. This passage mattered greatly for both the military and trade – on the town's east–west axis it held a rank comparable to the gates of the north–south axis.

While in post-Roman times trade increasingly moved along the Rhine and the western edge of the town remained largely unsettled at first, the old long-distance road survived below ground. As early as the last quarter of the 19th century, a well-preserved stretch of the westward military road was uncovered on the site Apostelnkloster 13–15; it was about 6.50 m wide. Its construction was carefully layered: a substructure of trachyte, above it a mixture of gravel and a course of brick concrete. Gravel layers show that the surface was renewed several times between the 1st and 3rd centuries AD.

During building work in 2009 – when the archive of the Fritz Thyssen Foundation, based at the Apostelnkloster, was extended – the old excavation site came to light once more. A cross-section through the road embankment, made from material found here and presented as a loan from the Roman-Germanic Museum, illustrates how the road was built up. This section, however, is inside the foundation's private building and cannot be viewed by the public.

One of the last traces of the gate appears in the Ottonian "Vita Brunonis": Ruotger describes the funeral procession of Archbishop Bruno, who died in Reims in 965 and was carried along the old Roman road into Cologne's suburb. A halt was made at a gate and a small church dedicated to the Apostles before the procession continued to the collegiate church of St. Pantaleon. From this, scholars conclude that the west gate was still in use in the 10th century. After that its traces are lost: the founding of the Apostles' collegiate and the construction of the church [[St. Aposteln]] around 1020/30 are thought to have blocked the gate, forced a rerouting of the road, and eventually caused the ancient gate to disappear entirely.