We're majorly not cool with these guys. If you remember correctly, they assault the bride:
Making their rounds in the city
the sentinels found me;
they beat me, they wounded me,
they took away my mantle,
those sentinels of the walls. (5:7)
Yep, they beat her, wound her, and undress her. But it doesn't get much screentime, does it?
We're guessing that the important part of this episode—in the context of the Song of Songs as a whole—is the fact that the city sentinels live in, well, the city. Compared to all the imagery in the rest of the poem, this passage provides a sharp contrast.
So why all the hubbub? Biblical writing is famous for mixing imagery from pastoral and urban areas—and it usually reveals the author's loyalties. Our educated guess? This author is a country guy.