The Song of Songs is a pastiche of songs and episodes. After the Shulamite woman sought and seemingly found her lover, he now echoes back to her a song that extols her physical beauty:
He compares her to the glory and beauty of royal cites: Tirzah in the north and Jerusalem in the south.
6:4 You are beautiful as Tirzah, my love,
comely [lovely] as Jerusalem,
Her beauty is like encountering theophany, creating a sense of dread (cf. 6:10) [this remains a compliment):

terrible as an army with banners.
The expression “army with banners” (nidgalot) mostly likely refers to celestial works or constellations or starry heavens rather than suggesting a military image. Or it could be more simple: decked in splendor. The next two lines explain why the woman is like a theophany — creating awe:
5 Turn away your eyes from me,
for they overwhelm me!
These are not images we would normally (today) use for extolling the attractiveness of the woman we love — like a royal city accompanied by heavenly hosts.
But, the point is important: as she distinguished him among ten thousands, so she stands out like the two great cities of the Land.