den of vice; rogue's den
A place where criminals, delinquents, or morally corrupt people gather; often used figuratively for a dangerous or lawless spot.
That bar was known as a den of criminals.
Specifically refers to a place of prostitution or a red-light area; cross-referenced with 売春宿.
See also: 売春宿
cluttered room; messy den
Colloquial use; describes a room that is extremely messy or chaotic, like a 'den' of clutter.
After the war, this street flourished as a red-light district.
His room is as messy as a den.