Paleomagnetic tests on two archaeological structures from the Crusader fortress Vadum Iacob (Ateret) reveal the use of these structures as an oven and a lime kiln. These fireplaces enable determination of the local direction and intensity of the geomagnetic field on 24 August 1179, when its construction was terminated by the Arab conquest. Stable field directions of the natural remanent magnetization were obtained in 31 specimens from a total of 16 building stones taken from the two archeological structures in the site. We consider the results as a negative stability test, indicating that a temperature above the Curie temperature erased the original in situ magnetization of each building stone when the stones were part of the fireplaces, and a new magnetization was acquired on the day the fire was extinguished. The mean magnetic directions of the two installations are statistically indistinguishable 013°/48°; α95 = 6.5° and 017°/58°; = α95 = 9.1°. The paleointensity of the Earth's magnetic field obtained by the original Thellier double-heating method from 8 basalt specimens from the lime oven is 72 ± 7 μT, about 1.8 times the current field. We demonstrate how the geophysical study of paleomagnetism helped resolve an archaeological question and how the archaeological and historical study helped resolve the dating of a geophysical feature.