Simplify the following text for a Grade 6 native English-speaking person.
Original: The outer wall is estimated to have required over 100,000 cubic meters ( 131,000 cu yd ) of travertine stone which were set without mortar held together by 300 tons of iron clamps .
Simplified: The outer wall was made from about 100,000 cubic meters ( 131,000 cu yd ) of travertine stone . This was held together by 300 tons of iron clamps .