The Thermenmuseum is built directly over the remains of a Roman bathhouse dating from around 100 AD, making it one of the most intact Roman sites in the Netherlands. Heerlen — then Coriovallum — sat on the main Roman road from Boulogne to Cologne, and the bathhouse served the garrison and civilian population of what was a significant stopping point on the empire's northern frontier.
The museum does an unusually good job of explaining Roman daily life rather than just presenting stones. Interactive displays, scale models, and well-chosen artefacts put the ruins in context. The hypocaust system — channels and pillars under the floor that circulated hot air — is particularly clear here, and gives a vivid sense of the engineering behind Roman comfort.
The museum is small and can be covered in about an hour. It is in the centre of town and easily combined with a look at the Glaspaleis a few streets away.