Around 300 A.D., the Romans began the construction of the Imperial Baths in anticipation of Trier becoming a seat of government for the Roman Empire in the north. The Baths were never completed and used as a bath because Emperor Constantine left Trier for Turkey before they were finished, so work halted.
The Roman baths were as much a focal point for social interaction as they were for health and exercise and they were very complex and expensive to build. We experiened the health part in Baden-Baden, but the baths in the modern world are no longer social centers.
The Baths were being built for the Emperor, so when he moved away, the expense was no longer justified. Years later, the Imperial Baths were completed as a military barracks, but they fell into ruin after the romans left in the later 400s.
Here are the ruins, to the extent they are above ground:
But the truly amazing part was all underground. The tunnels seemed to go on forever. This underground part was where all the plumbing, heating and sewage activities were to take place to keep the baths working.
And the construction techniques were truly amazing. Look at how they built the arches and how they alternated bricks with stone. The ancient Romans had figured out how to make cement and concrete and they used it expertly.