▁responsible ▁for ▁the ▁later ▁repair ▁of ▁Not ting ham ▁Castle ▁and ▁its ▁conversion ▁into ▁a ▁museum . ▁ ▁The ▁tunnel ▁is ▁approximately ▁ ▁in ▁length , ▁with ▁a ▁central ▁open ▁section ▁some ▁ ▁in ▁length . ▁It ▁was ▁intended ▁to ▁allow ▁access ▁to ▁and ▁from ▁the ▁park ▁by ▁horse - dra wn ▁carri ages , ▁and ▁the ▁original ▁requirement ▁was ▁for ▁a ▁tunnel ▁with ▁a ▁maximum ▁gradient ▁of ▁ 1 ▁in ▁ 1 4 ▁and ▁the ▁ability ▁for ▁coach es ▁to ▁pass ▁each ▁other ▁within ▁the ▁tunnel . ▁As ▁built , ▁the ▁tunnel ▁actually ▁had ▁a ▁gradient ▁of ▁ 1 ▁in ▁ 1 2 , ▁which ▁was ▁considered ▁too ▁great ▁for ▁horse - dra wn ▁carri ages , ▁and ▁as ▁easier ▁access es ▁were ▁constructed ▁elsewhere ▁in ▁The ▁Park ▁Est ate , ▁it ▁never ▁ful filled ▁its ▁original ▁purpose . ▁However ▁the ▁tunnel ▁is ▁still ▁in ▁use , ▁providing ▁ped est rian ▁access ▁to ▁and ▁from ▁The ▁Park ▁Est ate . ▁ ▁From ▁The ▁Park ▁Est ate ▁end , ▁the ▁tunnel ▁is ▁approached ▁from ▁T unnel ▁Road , ▁and ▁the ▁original ▁veh icular ▁nature ▁of ▁the ▁tunnel ▁is ▁clear ▁from ▁the ▁r amp ed ▁access . ▁The ▁first ▁section ▁of ▁tunnel ▁passes ▁under ▁New castle ▁Drive ▁and ▁the ▁Ro pe walk ▁and ▁is ▁un lined , ▁being ▁car ved ▁from ▁the ▁natural ▁Not ting ham ▁Castle ▁Sand stone . ▁An ▁open ▁section ▁in ▁the ▁centre ▁of ▁the ▁tunnel ▁provides ▁natural ▁light ▁and ▁vent ilation , ▁and ▁a ▁st air case ▁asc ends ▁around