In 1966, Castrillo made his debut in the Filipino arts scene with his first solo exhibition at the Northern Motors showroom in Makati. That same year, he unveiled his first significant public sculptures, namely, "The Virgin" at La Loma Cemetery and "Youth's Cry of Defiance" in Fort Santiago, both located in Metro Manila. Despite the Marcos dictatorship and the height of Martial Law during the 1970s, Castrillo was considered the most cutting-edge sculptor in the Philippines. By the 1980s, his reputation as a leading artist in the country was indisputable. He traveled extensively abroad for cultural visits, lecturing, and conducting research on the origins of early Filipino art.

Castrillo's primary medium was metal, particularly brass, bronze, and steel, which he transformed into sculptures through the process of hammering, cutting, and welding, with the assistance of a group of assistants. He also integrated other materials such as wood, plastic, plexiglass, ivory, and even neon lights into his works. His oeuvre consisted of abstract freestanding pieces, functional art pieces, body sculptures, liturgical art, and art jewelry.