Complex sentence: On the shore of Lake Gopło stands a medieval tower, nicknamed the "Mouse Tower"; however, it cannot be the site of the events described in the legend as it was erected some 500 years thereafter.
Split into simpler sentences: On the shore of Lake Gopło stands a medieval tower, nicknamed the "Mouse Tower". However, it can't be the site of the events described in the legend as it was erected some 500 years thereafter.