Complex sentence: In 462 the Romans officially ceded Septimania to the Visigothic king Theodoric II who had held Carcassonne since 453; he built more fortifications at Carcassonne, which was a frontier post on the northern marches: traces of them still stand.
Split into simpler sentences: In 462 the Romans officially ceded Septimania to the Visigothic king Theodoric II who had held Carcassonne since 453. He built more fortifications at Carcassonne, which was a frontier post on the northern marches; traces of them still stand.