Complex sentence: The abbey developed as a centre of study during the 12th century, and the 13th-century abbots Albert (1199-1217) and Theoderich II (1256-1295) added significantly to the buildings and architectural decoration, including the celebrated tomb of the founder.
Split into simpler sentences: The abbey developed as a centre of study during the 12th century. The 13th-century abbots Albert (1199-1217) and Theoderich II (1256-1295) added significantly to the buildings and architectural decoration, including the monumental tomb of the founder.