Complex sentence: The site of the former first Baltimore County and Town/City Courthouse torn down in 1809 was originally designated as the location for the newly planned Washington Monument designed by Robert Mills of which the cornerstone had just been laid on the 4th of July, 1815.
Split into simpler sentences: The site of the former first Baltimore County and Town/City Courthouse (torn down in 1809) was originally designated as the location for the newly planned Washington Monument. Designed by Robert Mills, the cornerstone of the Washington Monument had just been laid on the 4th of July, 1815.