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This is a small city in Belgium. The fortifications are a Vauban Fortress; but most parts of those medieval fortifications are now nature reserve (and protected as heritage). Some parts are open water, others are swamps.
However, most of the north-eastern side of the fortress was mapped as "forest" or something generic. After visiting it, I started micromapping this bit by bit. Upon saving and refreshing, the underlying structure of the fortress was suddenly visible, which was a nice surprise!
