It has served as a fortress for the Gallo-Romans, the Visigoths, Franks, and medieval French (you can see the layers of their masonry in the ramparts).
( It ( ( has ( served ( as ( ( ( a fortress ) ( for ( ( ( ( ( ( ( ( the Gallo-Romans ) , ) ( the Visigoths ) ) , ) Franks ) , ) and ) ( medieval French ) ) ) ) ( -LRB- ( ( you ( can ( ( see ( ( the layers ) ( of ( their masonry ) ) ) ) ( in ( the ramparts ) ) ) ) ) -RRB- ) ) ) ) ) ) . ) )
(ROOT (S (NP (PRP It)) (VP (VBZ has) (VP (VBN served) (PP (IN as) (NP (NP (DT a) (NNS fortress)) (PP (IN for) (NP (NP (DT the) (NNP Gallo-Romans)) (, ,) (NP (DT the) (NNPS Visigoths)) (, ,) (NP (NNP Franks)) (, ,) (CC and) (NP (JJ medieval) (NNS French)))) (PRN (-LRB- -LRB-) (S (NP (PRP you)) (VP (MD can) (VP (VB see) (NP (NP (DT the) (NNS layers)) (PP (IN of) (NP (PRP$ their) (NN masonry)))) (PP (IN in) (NP (DT the) (NNS ramparts)))))) (-RRB- -RRB-)))))) (. .)))
The fortress has been knocked down several times.
( ( The fortress ) ( ( has ( been ( ( knocked down ) ( several times ) ) ) ) . ) )
(ROOT (S (NP (DT The) (NNS fortress)) (VP (VBZ has) (VP (VBN been) (VP (VBN knocked) (PRT (RP down)) (NP (JJ several) (NNS times))))) (. .)))