Try Merida, Colonia, Bath, Rome (to a very substantial degree), there's a bunch of them.
Later, Medieval cities were frequently sited in the same areas as earlier Gaulish and Roman cities and towns.
Some, like an old Roman town in the middle of the military training area at Hoenfelz, Deutschland, never recovered ~ probably because it was stuck on the side of a hill.
You keep asserting this, but it isn't true. There are contemporary accounts. They just don't support what you want. For example, the De Excidio Britanniae had plenty to say about the ravening hordes of Saxon invaders causing destruction and famine. It was written (another example of those darn illiterates still stubbornly producing writing) in the 6th c., contemporaneously with the historical Battle of Badon Hill of Arthurian legend.
Yet, despite his obvious relish at describing the horrors of the Saxon invasion, Gildas makes no reference to a climatological catastrophe.
Later, Medieval cities were frequently sited in the same areas as earlier Gaulish and Roman cities and towns.
Yeah. Because they never stopped being occupied. Rome dwindled in size because it dwindled in importance following the Eastern Roman Empire and Byzantium, but it was never abandoned, even after being sacked. Paris, OTOH, grew. So did Aachen and Cologne.