Complex sentence: Numerous back-to-back houses, two or three storeys high, were built in Birmingham during the 19th century, the majority of them were still in quite good condition in the early 20th century and also prior to their demolition.
Split into simpler sentences: Numerous back-to-back houses, two or three storeys high, were built in Birmingham during the 19th century. Most were still in quite good condition in the early 20th century and also prior to their demolition.