Complex sentence: Before its erection this was the FM and TV-mast Wesel, which, after the demolition of the chimney, is once again the tallest structure in North Rhine-Westphalia.
Split into simpler sentences: Before its erection the FM and TV-mast Wesel took this claim. After the demolition of the chimney, the Wesel tower once again became the tallest structure in North Rhine-Westphalia.