On the roof above the entrance to the parade ground there is a square watchtower. Though completely remodelled, it bears distant testimony to the old beacon or lookout post, the earliest precedents to today’s castle. In 1777, two sundials, accompanied by an inscription, were mounted on the tower’s eastern and western facades. And a plaque at the base identifies this as the place where the French astronomer Pierre Méchain obtained the geographical coordinates of Barcelona, in 1792 and 1793, and established the trig point that would serve to measure the meridian from Dunkirk. The length of this meridian, linking Barcelona, Paris and Dunkirk, was used as the basis of the metric system.

Give the exceptional location of Montjuïc Hill, overlooking the sea, it made sense to place a beacon or lookout post there. The first mention dates back to 1073, which explains that during the day the lookout post warned of the approach of enemy ships by signalling with a system of sails, and at night by signalling with fire. The structure and function of this lookout tower were transformed by a series of modifications introduced between the 14^th and 17^th centuries.

It will not be until 1848 that it was once again used as a communications tower, with its incorporation into an optical telegraph network involving a system of pivoting cross members, still preserved today. This was used to send signals to the rest of the military posts around the city, such as those in Drassanes, the Ciutadella and the Captaincy General.