Not the original main entrance,it seems,it was removed to be replaced by elevated gun emplacements,to the rhs of this entrance.
see http://en.wikipedia.org/wiki/Tilbury_Fort
see http://www.castlexplorer.co.uk/england/tilbury/tilbury.php
see http://www.english-heritage.org.uk/server/show/nav.001001001013003003009