keep guard (verb)

1. Why did the scarecrow become a security guard? Because he wanted to keep guard over the corn field!

2. In the animated series, the superhero always keeps guard over the city to protect its citizens.

3. The androids in the movie 'Blade Runner' are programmed to keep guard over the secret replicant laboratory.

4. In the classic novel 'To Kill a Mockingbird', Atticus Finch keeps guard over the town's moral righteousness and fights against injustice.