Introduction to Neptune Fountain: Neptune Fountain is located at the north end of the square and was designed and built by Giacomo della Porta in 1574. The statue group has two themes. One is the sea god Neptune (called Poseidon in Greek mythology) fighting an octopus standing in the center; the other is the sea nymph Nereides who is accompanied by Cupid around the fountain. (Nereidi) and walrus.