Harvard Yard is the original campus — a cluster of red-brick buildings and great elm trees enclosed by wrought-iron gates that date back to the 17th century. Massachusetts Hall, the oldest surviving building, has been in continuous use since 1720. The Yard is open to the public and walking through it is free; the gate-guardians will happily direct you.
The centerpiece is the Statue of John Harvard, which stands in front of University Hall and is the single most-photographed spot in Cambridge. Widener Library, one of the largest university libraries in the world, dominates the south end. The Memorial Church on the north side is open for visitors and worth a look inside for the carved names of Harvard's war dead. The Yard is at its best on weekday mornings before the tour groups arrive.