Athena Promachos was one of two giant statues on the Acropolis. While Athena Parthenis stood in the Parthenon, Athena Promachos stood on a large plinth on the forecourt leading from the Propylea (gate to the Acropolis complex). The statue, by Phidias, was over 30 foot tall and gilded in gold. According to Pausanias, Athena held a spear whose tip sailors and travellers approaching from Sounion in Attica could see dazzling in the sun.
In her guise as Promachos, Athena was the defender of the city. Promachos translates as 'fighting in the front line.' Athena was the main deity of Athens.
You can read about the evolution of the Hellenic Portraits in the blog.