The appearance and structure of an ancient city wall made of earth might have been greatly changed beyond recognition through long exogenic or/and earth-leveling processes. It is therefore crucial to gain a clear idea of the original extension of the moat and the structure of the city wall in the protection of the city wall. The electric conductivity of a slacktip deposit is different from that of the original city wall, so is the filling in the moat course from the raw earth along the moat. Their difference in the resistivity is thus effective in distinguishing the structure of a city wall from the extension of a moat. This paper presents a successful case study of the wall and moat of Helv fortress, the result of which has been proved to be consistent with that obtained by an traditional archaeological method.