The Celts arrived in the wake of the Roman withdrawal at the end of the fourth century.
( ( The Celts ) ( ( ( arrived ( in ( ( the wake ) ( of ( the ( Roman withdrawal ) ) ) ) ) ) ( at ( ( the end ) ( of ( the ( fourth century ) ) ) ) ) ) . ) )
(ROOT (S (NP (DT The) (NNPS Celts)) (VP (VBD arrived) (PP (IN in) (NP (NP (DT the) (NN wake)) (PP (IN of) (NP (DT the) (NNP Roman) (NN withdrawal))))) (PP (IN at) (NP (NP (DT the) (NN end)) (PP (IN of) (NP (DT the) (JJ fourth) (NN century)))))) (. .)))
The Celts did not arrive until the start of the sixth century.
( ( The Celts ) ( ( ( did not ) ( arrive ( until ( ( the start ) ( of ( the ( sixth century ) ) ) ) ) ) ) . ) )
(ROOT (S (NP (DT The) (NNPS Celts)) (VP (VBD did) (RB not) (VP (VB arrive) (PP (IN until) (NP (NP (DT the) (NN start)) (PP (IN of) (NP (DT the) (JJ sixth) (NN century))))))) (. .)))