Set in the upper north-eastern corner of Barcelona and bounded by the Collserola mountains, the municipal district of Horta-Guinardó was mainly farmland and market gardens up till the beginning of the 20th century. A dozen or so barrios, from Horta in the north to Guinardó in the south were united by the building boom of the 1950s which transformed the area into an almost endless expanse of high-rise suburban apartment blocks. Apart from the odd mediaeval farmhouse and the vestiges of the 14th century Monasterio de la Vall de Hebron to the north of Sant Gens, there is little left of historical note. The Vall de Hebron is now synonymous with the neighbourhood that the city of Barcelona reconstructed to create four of the Olympic venues in 1992, providing the area with a swimming pool, tennis courts and football and rugby playing fields. Known as Barcelona's reservoir due to the large number of watercourses flowing down through the district from the hills of Collserola, Horta-Guinard also boasts a number of green areas including the oldest garden in the city - complete with maze - the 18th century neoclassical Parc del Laberint. There is a busy shopping area centred round the municipal market in Plaa Eivissa, where most shops are family-run. Two underground lines and several bus routes run through the district connecting it to the centre of Barcelona as well as to the neighbouring districts of Gàrcia and Nou Barris.