Due to land use changes, sandy open xerothermic areas in southern Germany have become rare, small, and isolated. Consequently, many xero-, thermo- or psammophile spider species have become restricted in their distribution, and endangered. The remaining open landscapes are priority sites for nature conservation. This also applies to the large (70 ha) former military site “Alter Flugplatz” within the urban area of Karlsruhe. There we studied the effects of management measures and habitat structure on the spider assemblages in three biotope types using one year of pitfall trapping. Ruderalized sites showed distinct assemblages from sandy turf and mat grass sites. The mowed mat grass sites differed from the grazed sites, which were more similar to the sandy turf. Apparently, habitat structure was more important for spider assemblages than plant species composition. Spider diversity correlated positively with vertical structural diversity, and negatively with bare ground in the trap surroundings. Grazing strongly increased the spider species richness in the ruderalized sites, but only slightly in grassland biotopes. Target species were more frequently caught under grazing. The spider guild composition was altered by grazing in sandy turfs and Nardus grassland, but not in the ruderalized sites, while guild diversity was always lower under grazing.