This room was used for several centuries as a reservoir-collector of rainwater from the parade of arms, the proof is in a hole that pierces the vault in the center like a skylight. Before that, during the Templar period, it may have been the castle's armory room.

The Guardhouse is formed by two outbuildings, the former guardhouse of the fortress was located on the lower floor of one of the gate's defensive towers. This dependence retains the trace of one of the fortress's primitive medieval stairs. From the first quadrangular floor of this annex, two more rooms are accessible, all covered with stone vaults. The castle, although of Gothic architecture, was built using techniques from 150 years earlier – the golden age of the Knights Templar.  

The greatest care of the Knights Templar was their personal weapons , established by the Rule: sword, spear and mace . In Peñíscola, according to an inventory of 1301, a large number of weapons were stored, such as crossbows, projectiles, shields, iron helmets and swords.

The Templars also distinguished themselves as cunning builders of war machines for launching projectiles, essential in the siege, assault and defense of fortresses. As is the case of the trebuchets (a siege weapon, dating from the Middle Ages, and widely used by the Templars) used in the defense of Miravet, which was inventoried in this same castle in Peñíscola.

As is well known, the Knights of the Order of the Temple were responsible for introducing some innovative aspects in military architecture, which will be progressively adapted and disseminated throughout the Middle Ages. Peñíscola Castle is an excellent example of this.