Complex sentence: Previously the Hall had been the ground floor area behind the front tower with windows onto the yard; when the new hall was opened this area was converted into three classrooms onto the yard, and laboratories to the front onto Queens Drive.
Split into simpler sentences: Previously the Hall had been on the ground floor behind the front tower with windows onto the yard. When the new hall was opened, this area was converted into three classrooms onto the yard, and laboratories to the front onto Queens Drive.