The Ubii Monument is the oldest stone tower in Germany, buried in the basement of a modern office block in Cologne's old town. A glass-fronted entrance on street level is the only outward sign of what lies below.

The tower was built no later than 6 AD, making it the earliest known evidence of Roman Cologne. Its foundations rest on oak logs driven into the ground near the Rhine, cut in the winter of 4 or 5 AD and preserved beneath the city for two thousand years. Originally twelve metres tall, six metres survive. When the Roman city wall was built at the end of the first century, the tower was incorporated into it.

The Ubii were a Germanic tribe who settled here alongside Roman veterans after Julius Caesar conquered Gaul in the 50s BC, giving the monument its name.
The tower opens only on the first Thursday of each month.

VoiceMap's self-guided audio tours trace the full line of Cologne's Roman wall, placing the Ubii Monument at the beginning of the city's story.