This article analyses the walled defensive system of the Khmer city centre of Vimayapura (modern Phimai, Thailand) to evaluate the theoretical level of military effectivity of both the walls and the moats against potential attackers, considering their technical characteristics and the enemy’s weapons. We also study the layout of the urban enceinte, the constructive material, the gateways as well as weakness and strengths of the stronghold and the symbolic, monumental and ornamental functions in the overall role of the walls. Based on comparisons with similar cases, as well as in situ observations of the archaeological remains and a bibliographical research, our study reveals that the stonewalls were not designed primarily to resist military attacks. Instead, the army, the moat, and possibly the embankments and/or palisades would have been the first lines of defence of the city.