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 arrived during the Roman withdrawal.
( ( The Celts ) ( ( arrived ( during ( the ( Roman withdrawal ) ) ) ) . ) )
(ROOT (S (NP (DT The) (NNPS Celts)) (VP (VBD arrived) (PP (IN during) (NP (DT the) (NNP Roman) (NN withdrawal)))) (. .)))