The museum was created in 1917 by the Pro-Mount Society. At first the association would use an establishment in the town of A Guarda. In 1953 the museum was moved to the current building, at the fort hill site on top of Santa Tegra hill.

Regarding the age of the castro, people agree that the site timing is between the 1st century BC and the 2nd century AD, at least at the site's peak. That is the timeline of most of the pieces exhibited in the museum. Nevertheless, later excavation campaigns in the fort hill and in the surroundings brought to light elements from other periods: Palaeolithic, Neolithic and Bronze Age. These pieces - included in the museum collection - show that the mouth of the Miño River was already inhabited since time immemorial.