On February 3, 2023, I worked alongside a small group of archaeologists to conduct a Ground Penetrating Radar (GPR) survey of the medieval castle of Zorita located roughly an hour northwest of Madrid. Our small team of archaeologists were joined by Jorge Padin Devesa and Sergio Navarro Martinez from the Polytechnic University of Valencia in Spain. Jorge and Sergio are the GPR experts – the individuals with the high-tech toys and the knowledge to conduct this survey.

Buoyed by stories we first heard from Dionisio, the deceased former Mayor of Zorita, of how during his childhood he played hide and seek with his friends in the castle ruins, we began the survey with high expectations. In one of our most promising survey locations, he shared there was a cellar with large vessels (tinajas) scattered about the room. For the past few months we waited patiently (not really) for the results. GPR studies take time to compile and analyze the data into something meaningful. While it starts with a survey on the ground that lasts but a day or two, the real work is completed by experts intently studying a series of wiggly lined charts and somehow identifying anomalies.

To recap, GPR is an indirect, non-destructive electromagnetic prospecting technique that utilizes electromagnetic waves emitting from a transmitting antenna and directed into the ground. These waves generate reflections off objects in the subsoil which are received and recorded by a receiving antenna. The output of a survey is presented in the form of radargrams – graphs with the x axis corresponding to the distance covered by the pulses and the y axis representing the depth of an item. Amplitudes are impacted by the material in the subsoil. To the trained eye, it is possible to use these graphs to identify the position and dimension of items in the earth several.

Back in February, the onsite archaeologists Catalina, Dionisio, and Mat Saunders, helped the GPR experts identify high potential areas based upon prior excavations and the local lore. Figure A identifies the zones as they were identified on the castle grounds.

Figure A – Overview of Targeted Locations

Now for the fun part – the results. In nearly all the zones, some form of anomaly was located. However, the results in three areas are particularly interesting.

Zone 1 – West Entrance

This survey focused on the area immediately inside the southern gate of the castle – the very area mentioned by the former mayor. The radargram in Figure B shows a view of the results. For a GPR survey, a series of radargrams are delivered for each location. By overlaying them, it is possible to get a clearer picture of what lies below ground. Based upon the reflections in this area, there is an underground structure with a vaulted (perhaps barrel shaped) roof. This room is rectangular and long and is likely some form of cellar. The ceiling of this structure is only one meter underground. An anomaly was detected that could very well be the entrance.

In viewing the radargram, an orange line indicates the areas where a hyperbola shaped reflection indicates the roof of the structure. To the side is a blue rectangle that may indicate an entrance to the room.

Zone 3 by the East Wall

The survey of Zone 3 delivered the biggest surprise overall. This location is not far from the eastern wall of the castle and just a short toss from current excavations. The GPR results identified a large underground cavity. This room appears round and covered with a dome – a bit like an igloo shape. The reflections did not yield any clues on where the entrance might be. For a structure as large as this one, it is uncanny there were no stories or clues about its existence.

One of the most interesting radargrams of this room is shown below in Figure C. The orange rectangle is a single inversion that indicates a vaulted structure. The sides of the profiles are very well defined in this picture. The yellow rectangle to the side indicates an inversion of the signal. Although the signal is lost immediately to the side of this inversion. This may indicate a portion of the structure collapsed. The green area appears to be some form of filling zone. A multitude of small reflections indicates a large-scale disturbance of the soil.

Zone 5 – By Known Cemetery

The radargrams for Zone 5 identify an anomaly roughly 60 cm below the surface. The signal is clearly inverted so it is some type of cavity and it may well be a tomb. In the immediate area, there are further similar inversions which are likely also tombs. These areas are just off from the cemetery but well beyond the known limits of burial plots. These inversions are at significantly different levels below the surface which is not uncommon for burials during the time the castle was occupied.

Figure D below is a comprehensive view of the interesting features discovered as a result of the February survey. As a result of the survey, our process moving forward adjusts. For the next few seasons, exploratory digs will be focused on the promising locations identified during the survey with the goal of locating entrances and getting inside these structures. Perhaps that is what makes Zorita Castle so interesting to our group. Slowly over time it is giving up its secrets – but there always seems to be another find just around the corner. If something is clear, it is that the castle was almost as big under the ground as it was above - cellars, cisterns, circular chambers, silos, dungeons, crypts, wells and burials...

Time to start digging!

Zorita Castle Part 3 of 3: The Findings