Prison in the middle of a forest?

Is this a secret prison? Perhaps of the German secret service?
Nah, these are the two entrances to the elevated tank of the village Lichtenberg. It can contain 250 cubic metre of drinking water.
In ca. 1890 Lichtenberg got a water pipe to Lützelbach, the village higher up in the valley. Until then the villagers had to get either the water out of a local well, or get it from other places on donkeys.
In  ca. 1926 the pipe was renewed, and at the same time the elevated tank built. In the decades between then and now the tank was renovated two, three times, and extended to end with the current format.
The position of the tank is high enough to ensure clear drinking water with enough pressure for the village.