Complex sentence: Prior to Fort Darland becoming a detention centre it was an "Army Technical school" for (boys) built in 1938 and opened for its first intake in March 1939 it served as a school for both Royal Engineer and Royal Artillery enlisted boys.
Split into simpler sentences: Prior to Fort Darland becoming a detention centre it was an Army Technical school for boys, built in 1938 and opened for its first intake. In March 1939 it served as a school for both Royal Engineer and Royal Artillery enlisted boys.