Gothic architecture, usually churches or university buildings, continued to be built. Ireland was an island of Gothic architecture in the 17th and 18th centuries, with the construction of Derry Cathedral (completed 1633), Sligo Cathedral (c. 1730), and Down Cathedral (1790–1818) are other examples. In the 17th and 18th century several important Gothic buildings were constructed at Oxford University and Cambridge University, including Tom Tower (1681–82) at Christ Church, Oxford, by Christopher Wren. It also appeared, in a whimsical fashion, in Horace Walpole's Twickenham villa, Strawberry Hill (1749–1776). The two western towers of Westminster Abbey were constructed between 1722 and 1745 by Nicholas Hawksmoor, opening a new period of Gothic Revival.