Three small ‘high-level’ water tanks are located on the surface, one by the Pump House and two on the terreplein above the West Postern Gate. Tanks at this level would be required to maintain an adequate head of water to supply surface buildings. The completion of the Citadel during the 1850s and 1860s explains the earliest high-level tank, dated 1861. Others followed in 1891 and 1901, also periods of rapid expansion of surface facilities. All the tanks are constructed from cast-iron flanged panels bolted together.(location accurate to the nearest 1m based on available information)