It seems Gibraltar is a very important symbol for the Spanish:

The Pillars of Hercules is the ancient name given to the promontories that flank the entrance to the Strait of Gibraltar. They are Gibraltar in Europe and Monte Hacho in Ceuta in Africa. The Jebel Musa, west of Ceuta, in Morocco, is sometimes considered one of the Pillars.

The Pillars of Hercules has its origin in Greek mythology as the Pillars of Heracles which is the indefinite name of the ancient Greek hero Hercules (Which is also the more famous name of the mythical hero).

The naming of the pillars

When Hercules had to perform twelve labours, one of them was to fetch the Cattle of Geryon in Spain and bring it to Eurystheus. On his way to the island of Erytheia he had to cross the mountain that was once Atlas. Instead of climbing the great mountain, he cut corners and put his mind to work. He decided to use his great strength to smash through the colossal mountain that used to be a colossal giant. Hercules split it in half using his indestructible mace or club (Myths vary). By doing so, he connected the Atlantic Ocean to the Mediterranean Sea and formed the Strait of Gibraltar. One part of the split mountain is Gibraltar and the other is Monte Hacho. These two mountains taken together have since then been known as the Pillars of Hercules or Heracles.

The pillars as portals

The pillars are also mentioned at some places as portals, or gates to different locations on Earth. When the Carthaginian admiral Himilco was sent to explore the area of the Muddy Sea (a shallow plateau that lies to the southwest of the Pillars) his report included the words "Many seaweeds grow in the troughs between the waves, which slow the ship like bushes {...} Here the beasts of the sea move slowly hither and thither, and great monsters swim languidly among the sluggishly creeping ships" (Rufus Festus Avienus) This description accurately resembles the Sargasso Sea rather than the Muddy Sea.

The Pillars appear as supporters of the coat of arms of Spain. The motto Plus Ultra (Latin for further beyond) indicates the desire to see the Pillars as an entrance to the rest of the world rather than as a gate to the Mediterranean Sea, as opposed to Non Plus Ultra (nothing further beyond), the phrase inscribed in the mythological columns indicating their antique condition of border of the known world. It also indicates the overseas possessions that Spain once had.