ENCI exists since 1921 in the Dutch part of the hill. The quarrying of limestone in a protected nature area was controversial for many years but for economic reasons ENCI was able to extract most of the mount's limestone, creating a vast area of connected quarries. In 2010 it was decided that by 2018 the quarry will close down. Various plans for redevelopment of both the quarry and the factory site have been presented in recent years. Parts of the quarry that were abandoned years ago have been taken over by nature once again. Ecology Mount Saint Peter is a Natura 2000 protection area. Because of its limestone soil, the hill is exceptional in its botanical variety. It is the northern limit for a number of species of orchids due to its favorable microclimate. Some of the orchids found there include: Aceras anthropophorum, Ophrys apifera, Ophrys insectifera, Orchis militaris, Orchis purpurea and Platanthera bifolia. An abandoned part of the ENCI quarry has become in recent years the breeding ground of the Eurasian eagle-owl, locally known as oehoe (pronounced: uhu). Mount Saint Peter supports the richest environment for bats in Benelux, as well as the richest environment for orchids in Benelux. The Dutch part of the plateau is largely owned by the Dutch society for preservation of nature monuments Natuurmonumenten. A sheep herd is used to keep vegetation open in certain areas. Listed buildings Castles and farmsteads On the western part of the mount remains of the motte-and-bailey castle De Tombe can be detected in the landscape. On the northeastern section, now overlooking the ENCI quarry as well as the river Meuse, is the farmstead Lichtenberg, largely dating from the 18th century but which includes an 11th-century keep. Further south, in the Walloon municipality of Visé, is another ruined castle overlooking the Meuse, Caestert, of which only some of the farm buildings remain. Well-preserved is the 16th-century house De Torentjes ("The little towers"), at the bottom of the hill in the village of Sint Pieter. Other buildings The imposing fortress Sint-Pieter on the northern edge of the hill overlooking the city of Maastricht was built in 1702 as part of the defense works of the heavily fortified city of Maastricht. The fortress was recently restored to reflect its original state. The old village of Sint Pieter was part of the Prince-Bishopric of Liège until 1794. The medieval church was replaced in