Which preposition to use with sentinal

before Occurrences 1%

BATES (1 syl.), a soldier in the army of Henry V. He with Court and Williams are sentinals before the English camp at Agincourt, and the king disguised comes to them during the watch, and talks with them respecting the impending battle,Shakespeare, Henry V. Bates (Charley), generally called "Master Bates," one of Fagin's "pupils," training to be a pickpocket.

Which preposition to use with  sentinal