The trees 'stand sentinel' [=stand in a row like soldiers] on the cliffs.
armed 'sentinels'
(..) - Did Zion send the warning? - No, another ship. Shit. - Squiddy's sweeping in quick. - Squiddy? A sentinel. A killing machine designed for one thing. Search and destroy. Set her down there. How are we doing, Tank? Power off-line. (..)1999 The Matrix