Castleman served as the head football coach at the University of Colorado at Boulder in 1906 and 1907, compiling a record of 7–6–4. He was also the head basketball coach at Colorado in from 1906 to 1912, tallying a mark of 32–22, and the head baseball coach at the school from 1907 to 1913, amassing a record of 30–17. He was later the track coach at Ohio State University, where his team won the 1929 NCAA Men's Track and Field Championships.
Castleman died at his home in Columbus, Ohio, on October 9, 1946, at the age of 69.[3]