Yes, in both the specific and the general.

Apart from quibbles over what is meant by "Europe" (i.e. the Byzantines maintained a number of their strongholds in the Balkans well into the medieval era), there is documentation of Roman forts and buildings being used for more than cheap stone. The early British chronicler Gildas (fl. 6th century CE) writes about Roman fortifications extant and occupied during his lifetime, and noted modern historian Jim Bradbury follows the example of Bernard Bachrach in describing Merovingian warfare, at least, as being fought over surviving Roman fortifications.

Hadrian's Wall itself may have yielded protection to Britons after the Roman period. While considerable numbers of forts and outposts were abandoned in the interior, a surprising number of archaeological sites along the Wall show signs of both human occupation and attempts at re-fortification after it is documented that the legions had left. These fortified towns and buildings may have been in good use at least until the 6th century, confirming the account of Gildas. What is interesting is that artifacts in these post-Roman settlements occasionally display Roman characteristics, but also in many cases show Germanic elements. The history of Anglo-Saxon arrival in Roman Britain is shrouded in a touch of myth, but it is commonly believed that at some point an invitation was given to them requesting their aid as mercenaries (see the narrative of Vortigern, which...ends poorly). It is plausible that the Wall was maintained to protect post-Roman Britain from marauders up north, since the Picts and their like never really went away. Within Britain, at least, Roman fortifications seem to have been used for awhile -- at least in the North and along the Saxon Shore.

For further reading:

Bradbury, Jim. The Medieval Siege. Woodbridge: Boydell & Brewer, 1992.

Dark, K. R. “A Sub-Roman Re-Defence of Hadrian's Wall?” Britannia 23 (1992): 111-120.