expression, ichidan verb
set a watch; post guards
Used when assigning someone to keep watch or stand guard, often in a security or surveillance context.
敵の動きを探るため、見張りを立てた。
We posted a lookout to observe the enemy's movements.
見張る is the base verb meaning 'to watch' or 'to keep an eye on'; 見張りを立てる is a set phrase meaning 'to post a guard' or 'to set a watch'.
Compound of 見張り (watch, lookout) + を + 立てる (to set up, to post). The exact historical derivation is uncertain, but the phrase is conventionally used for assigning a guard or lookout.