Oxford's most unusual college has no undergraduate students. It only admits fellows, who are assessed by "the hardest exam in the world" (though it may have become easier since 2010, when it abandoned its three hour, one word essay paper: sample topics included "beauty" and "innocence").
Visitors welcome from 2-4pm on weekdays and Sundays when the college is open (closures in August, at Easter, and over the Christmas break). Free admission.