Arms of many medieval cities are based on seals that preceded them.
The mural crown is the symbol of the city and usually it replaces the
helm and crest. Its masonry and towers represent the wall that encircled
the city during the middle ages.
In Portuguese heraldry the number of towers in the mural
crown indicate status: “vila” (town) has four towers, “cidade” (city) has
five.
In rare cases the mural crown is put between the helm and the crest.