The Phoenix Park is the largest urban park in Europe and contains a number of stately homes, including the official residency of the President of Ireland, Áras an Uachtaráin, and the residency of the US Ambassador.The Phoenix Park is also home to a large herd of fallow deer. The deer have been present in the park since the 17th century, when they were hunted for sport by the gentry of the day. Phoenix Park is open to the public all year round. Contained within the grounds are a number of sports fields and there are also a number of cycle and walking routes through the park. The park also houses Dublin Zoo, reputed to be the fourth oldest zoo in the world. An exhibition on the history and wildlife of the Phoenix Park is on display in the Visitor Centre. Here visitors can receive information and view a historical interpretation of the park from 3500 BC to present day.Temporary exhibitions are also regularly on display in the centre. Adjoining the Visitor Centre is the fully restored Ashtown Castle, a medieval tower house that dates to before the 17th century. The castle had been incorporated into an 18th century mansion and was rediscovered when this building was demolished due to dry rot.