Summary The problem of rising moisture in historic walls is studied in terms of detection of the affected areas by GPR and ERT prospecting. The tests were carried out at the wall of a tower of the Heptapyrgion (Akropolis) fortress of Thessaloniki. The GPR survey was carried out employing 250 and 500MHz antennas along parallel transects of the wall. 2D GPR models were constructed and their response was computed to aid the attempted interpretation of the actual data. ERTs were also carried out along the same transects and the results compared to those of GPR. In general both methods agree with respect to the detection of areas of increased moisture content. Further, the condition of the interior of the walls can be assessed.