These native cities or citadels were characterised by their size and by urban features such as paved streets equipped with channels for stormwater runoff, reservoirs of potable water, and evidence of urban planning. Many of them also presented an inner and upper walled space, relatively large and scarcely urbanised, called "acrópole" by local scholars. These oppida were generally surrounded by concentric ditches and stone walls, up to five in Briteiros, sometimes reinforced with towers. Gates to these oppida become monumental and frequently have sculptures of warriors.