Ensuring a water supply was a distinctive problem for fortifications wherever their primary role was their strategic and military relevance. The fortification in Mosty u Jablunkova, the Great Fort, bulit in the challenging terrain of the Beskydy mountains, also faced the issue of ensuring the water required to maintain its defence potential. Relying on all the sources available (written documents, cartographic and material evidence), the article tries to settle the problem of a possible water pipe assumed to convey water from a distant source. Both written and cartographic evidence fails to give a clear and definite answer to this problem. However, a wood fragment of a water duct found near the Great Fort seems to cast a different light on the issue. Combining all these sources allows the existence of a water duct to the Great Fort to be proven, with a dendrological time analysis attributing it to between 1724–1729.