Christ Church is the grandest and most famous of Oxford's colleges. Founded by Cardinal Wolsey in 1524 and refounded by Henry VIII in 1546, it combines a college, a cathedral, a world-class art gallery, and some of the finest architecture in the city in one compound. You enter from St Aldate's through the Tom Tower — Christopher Wren's famous bell tower that houses a huge bell called Great Tom, which has rung 101 times every night at 9:05 since the 17th century.
The college is open to visitors who pay an entry fee. The main attractions are the Great Hall, the cathedral (a genuine medieval cathedral that doubles as the college chapel), and the college gardens that stretch along the Thames. The Christ Church Picture Gallery, housed in a separate building off Oriel Square, holds a strong collection of Old Master paintings and drawings, including works by Tintoretto, Van Dyck, and Michelangelo. The whole visit can easily fill half a day, especially if you include a walk along the meadows to the river.