I remember the Cellar Door in the mid '60s -- went there several times, but the only performer I really remember seeing there is Donal Leace. I was pleased to see him a few years ago at the Washington Folk Festival -- glad to know he's still around and performing.
A friend of mine in those days lived in a townhouse just a couple of doors up the hill (was it 29th St.? 30th St.?) from the Cellar Door on the corner of M St. in Georgetown. Several times, when we had music parties at her house, total strangers would drop in -- they'd been in line for the Cellar Door (a very small venue), but heard the music coming from the house and decided we were a better (or at least quicker and cheaper) deal.