Endearingly known as the "psychopath", Alleenberg is one of a little handful of bars in Frederiksberg that cater to a younger, more financially restrained crowd. Part of the allure is definitely the late opening hours all through the week.
This is one of the older bars with its original interior preserved, as per the edict of a former owner, who loved the place so much she would not see it destroyed. So you get a good mix of old timey charms as well as (fairly) cheap drinks.
Bring some piano skills for an impromptu session at the bars piano - it's a great way to meet some locals.
Tu-Sa: 21:00-05:00