The concentration of soluble salts in the retaining wall of the Wawel Castle was estimated. This wall bears the pressure of a steep, earthen slope of medieval fortifications. The analyses have shown that the samples contain mainly sulphate, nitrate, chloride, calcium, sodium and potassium. The concentration of the ions increases distinctly from the inner side of the wall toward the outer, exposed part. Due to the specific position of the wall and in contrast to most other buildings, water penetrates the retaining wall from the side abutting the scarp, while capillary suction from the foundations is of minor importance.