Simplify the following text for a Grade 6 native English-speaking person.
Original: The North Tower ( 1 ) was completed in and the South Tower ( 2 ) was finished in ; the two were collectively known as the Twin Towers , the two tallest buildings in Manhattan at that time .
Simplified: The North Tower ( 1 ) was completed in December 1970 and the South Tower ( 2 ) was finished in July 1971 .