Ho Ace,
Build one, no. That thing looks HEAVY. Notice the wheels for elevation and windage. Also, note the fairly crude sight along the right side of the gun. I think that thing was probably sighted in ahead of any anticipated action. I suspect that it wasn't used much cuz the river traffic would have paid the toll promptly.
Hello LR,
Knowing the Germans, I'll bet that the gun has been decomissioned but maintained and that it could be put back in action. But, what for? It makes more money as part of a tourist attraction. I've attached a map of the fort. The cannon was near the white button on the East side. The map is taken from the East looking West.
DD-DLoS