The dungeon of Tronchón is located in the main square of the town. It is the only example in the region located in a free-standing building, built for this purpose. At the entrance an ogee arch marks the arms of the town, which consisted only of a castle at the time of its construction. It is the oldest preserved coat of arms of Tronchón. The ogee arch and the half-barrel vault built in ashlar stone lead us to a medieval chronology, as well as its double door: the first reinforced with iron plate and the second of wood, which was lowered from the upper guardhouse.
Inside, the floor is the rock on which the building is built and there are no openings through which to see the light. In the background, it retains the heavy iron chain attached to the wall to limit the movement of prisoners.
In such dismal conditions, some of the prisoners had something to heat themselves with, so the walls and vaults were covered with soot. The resulting charcoal, together with stones and other improvised tools, allowed the prisoners to draw numerous symbols of their captivity. Hands, birds, horses, buildings, religious elements or a good collection of firearms, reveal the condition and aspirations of those who were imprisoned in the place.
Tronchón had another prison of later construction, the new jail located on the hill, next to the fountain and washing place. The two-story building remains, with a semicircular arched doorway.
The existence of prisons to lock up criminals was common in all the towns of the Bailiwicks, as the commanders and bailiffs of the Order of the Temple in Aragon enjoyed full civil and criminal jurisdiction in their territories.