Circus Bar and Billiards is located in the Cavern Club space (4 clubs for the price of 1 -- see the link above). This is an immensely popular bar, always packed with people having a good time. Lots of dancing, great music, karaoke on Fridays... If you love it wild and crazy, Circus is the place for you to spend your weekends.
Sometimes they even have live bands. The beers are crazy cheap (50 cent Pabst Blue Ribbon anybody?), and you can get your fill of the free popcorn. Shoot some pool, watch some music videos... and enjoy the classic entrance through the fire escape. You have to see it to believe it.