They are built on the site of David's Tower, once the largest and most formidable structure in the castle.
( They ( ( are ( built ( on ( ( the site ) ( of ( ( ( ( ( David 's ) Tower ) , ) once ) ( ( the ( ( ( largest and ) most ) ( formidable structure ) ) ) ( in ( the castle ) ) ) ) ) ) ) ) ) . ) )
(ROOT (S (NP (PRP They)) (VP (VBP are) (VP (VBN built) (PP (IN on) (NP (NP (DT the) (NN site)) (PP (IN of) (NP (NP (NP (NNP David) (POS 's)) (NNP Tower)) (, ,) (ADVP (RB once)) (NP (NP (DT the) (ADJP (JJS largest) (CC and) (JJS most)) (JJ formidable) (NN structure)) (PP (IN in) (NP (DT the) (NN castle)))))))))) (. .)))
There is nothing where David's Tower used to be.
( There ( ( is ( nothing ( where ( ( ( David 's ) Tower ) ( used ( to be ) ) ) ) ) ) . ) )
(ROOT (S (NP (EX There)) (VP (VBZ is) (NP (NP (NN nothing)) (SBAR (WHADVP (WRB where)) (S (NP (NP (NNP David) (POS 's)) (NNP Tower)) (VP (VBD used) (S (VP (TO to) (VP (VB be))))))))) (. .)))