Simplify the following text for a Grade 11 native English-speaking person.
Original: 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 .
Simplified: In 462 the Romans officially left and the Visigothic king Theodoric II built more fortifications at Carcassonne , some of them still stand .