Such cemeteries were established at high elevations along ridgelines and on hilltops. The Crusaders built fortresses on strategic hilltops. Celtic shrines were situated in remote areas such as hilltops, groves, and lakes. Watchtowers were also erected on some of the hilltops around the town. Some of the hilltops have plains which are interspersed with lakes. Geographic features include limestone and dolomite glades, which are rocky, desert-like areas on hilltops. To its north stood the forts Loreto and Guadalupe on opposite hilltops. Populations left the cities and moved to fortified villages on the hilltops. Quartzite is very resistant to chemical weathering and often forms ridges and resistant hilltops. This method usually leaves the ridge and hilltops as flattened plateaus. Settlements draw more closely together and tend to be sited on hilltops. Relief between valley bottoms and hilltops is several hundred feet. Hilltops, the so-called "hillforts", "castra" and monasteries have been excavated. Shrubs and trees fringe most drainage lines and cover hillslopes and hilltops. Mating often occurs in swarms, generally at landmarks such as hilltops.