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When the Spandau Citadel was built in 1559, a small canal called the Zitadellengraben (German for citadel ditch) was dug for two purposes:
- to serve as a moat for the fortification, and
- to ship supplies from Spandau via river Havel directly to the Citadel.
With walls up to 12 feet thick and a rampart before the wall, the Citadel was one of the most heavily fortified buildings at its time in Europe.
To get the supplies directly into the Citadel, a tunnel underneath the fortification was constructed on the north side of the fortress. In some maps, the exit from the tunnel on the inside is called Wassertor (German for water gate), in other maps it is simply called Hafen (German for harbor).
In times of danger, the tunnel was closed with solid iron gates. When Swedish invadors besieged the fortress in 1675, the tunnel was actually filled with dirt and the Swedes never took the Citadel.
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