The Kaiserthermen were meant to be the grandest bathhouse in Roman Gaul, and the builders ran out of steam before they finished. Begun in the early fourth century, probably under Constantine, as part of the imperial palace quarter, the complex was never completed as a bath, and instead was converted into a barracks, then a fort, and later part of the medieval town wall. What remains is a set of enormous brick walls, apses and arches that rise high above the surrounding lawns, with a facade that is unmistakably Roman in its scale.
The best part is underground. Beneath the bath halls a network of service tunnels, built to let slaves feed the furnaces and carry fuel and water without being seen by the bathers, still survives, and you can walk through them, bending your head in the dim light under arched brick ceilings. It gives a rare sense of the machinery behind Roman luxury, where heat from wood fires flowed through hollow floors and walls to warm the rooms above. The baths belong to the UNESCO group of Roman monuments in Trier, and a small entrance fee is charged, with opening hours that vary by season.
A short walk away lie the older Barbarathermen, which were larger still in the second century but were largely dismantled for building stone, so the Kaiserthermen are the better place to appreciate the scale.
For the rider, Trier is a place to park and walk. The centre is compact, the old streets are busy and partly closed to traffic, and the best approach is to leave the bike in a car park near the Moselle and stroll. The city has everything a traveller might need, from fuel to supermarkets to cafés, with a hundred thousand inhabitants. The ride out is rewarding, whether along the Moselle toward Luxembourg, with vines on one side and the river on the other, or into the Saar valley, where quieter roads lead past small wine villages.
Summer weekends fill the Moselle road with cyclists and camper vans, so a weekday morning is a better choice. Autumn brings the grape harvest and golden light. If you have an hour to spare, the Rheinisches Landesmuseum, close by, holds mosaics, tombstones and a famous carved relief of a wine ship, which captures the spirit of the Roman Moselle better than any guidebook.