IDIOMS

keep watch

(idiomatic, intransitive) To guard; to watch over someone or something.

Example: 1900, L. Frank Baum, The Wonderful Wizard of Oz
  In the afternoon the sun shone hot in their faces, for there were no trees to offer them shade; so that before night Dorothy and Toto and the Lion were tired, and lay down upon the grass and fell asleep, with the Woodman and the Scarecrow keeping watch.