Complex sentence: Between the summer of 2004 and the spring of 2006, the passageway in the basement was extensively modernised and adapted to become part of the Niki-de-Saint-Phalle-Promenade from Kröpcke to Raschplatz.
Split into simpler sentences: Between the summer of 2004 and the spring of 2006, the passageway in the basement was extensively modernised and adapted as a promenade. It thus became part of the Niki-de-Saint-Phalle-Promenade from Kröpcke to Raschplatz.