Ninety-seven alien plant species were found in habitats provided by rocks and walls around 56 castles in Germany. Compared to Central European agriophytes (i.e. alien species naturalised in natural vegetation), the flora adjacent to castles had more species introduced before 1500. More alien species introduced earlier were found at castles built earlier compared to castles built later. Fifty percent of the species introduced before and during the Middle Ages were usable plants at that time, compared to 20% of usable plants among the native species in the same habitats. Species introduced earlier were mainly used for medication and nutrition, whereas later introductions were used for ornamental purposes. Compared to the total alien flora of Germany, there were more useable species at the castles, nearly the same percentage of ornamentals and fewer plants without use. These results imply that the species composition of alien species growing in the vicinity of medieval castles today may be traced back to the historical reasons for their introduction to these places.