"Stand guard" is a phrase often used to describe the act of maintaining security or protection over someone or something. There are numerous synonyms that can be used to express the same sentiment, such as "keep watch," "stay vigilant," "remain alert," "be on the lookout," "protect," and "defend." Other similar phrases that can be used include "stand sentinel," "keep a lookout," "keep sentinel," "guard the premises," and "man the post." Essentially, any phrase or word that conveys the idea of monitoring and protecting can be used in place of "stand guard," depending on the context in which it is used.