the urban structure consists of a group of architectural elements, such the former-palace of monfalim and terena in the old quarter of porto. to the south is situated the public gardens of the "palácio de cristal" ("crystal palace") where the former-"torre da marca" was situated. the "torre da marca" was the landmark that indicated the terminus to navigation, constructed by king d. john iii in 1542 to orient ships along the douro, and was erroneously assigned to the "torre dos terenas" or "torre de pedro-sem".