Ancient Mantineia, located in the centre of Peloponnese, Greece, consists of an almost 4Km in length elliptical fortification wall containing 10 gates and 126 towers. Since the 19th century, a number of watchtowers have been located at the surrounding slopes of the Mantineian plain and the existence of a defensive network has been suggested. GIS analysis has been used specifically for studying the defensive network of the ancient settlement and its potential coverage. The gates of the fortification walls and the watchtowers were mapped through high precision GPS units. These data were superimposed on a DEM created by the digitization of 20m elevation lines covering a region of 1000 Km2. Geological maps, aerial and SPOT satellite imagery were also incorporated to the GIS analysis. Viewshed analysis was performed in order to determine the extent and degree of intervisibility between the watchtowers situated in the nearby hills and mountains and the gates of the city walls. The data were correlated to the results of previous surface surveys that had located ancient roads and paths. Cumulative viewshed analysis was also employed to suggest the number of the defence constructions that were able to control a specific area. In this way, a thematic map of the potential defensive region has been created, suggesting either possible gaps or candidate archaeological sites that may have been used to complete the defence system of the region. The results of the analysis were superimposed on a SPOT Panchromatic image draped on the 3D DEM. Finally, a fly-over simulation in the area of interest was produced serving as a visual verification of the above analysis.