Complex sentence: Founded in the 10th century, the Rila Monastery is regarded as one of Bulgaria's most important cultural, historical and architectural monuments and it is a key tourist attraction for both Bulgaria and Southeastern Europe as a whole.
Split into simpler sentences: Founded in the 10th century, the Rila Monastery is regarded as one of Bulgaria's most important cultural, historical and architectural monuments. It is on account of this also a key tourist attraction in Bulgaria and Southeastern Europe as a whole.