Him Law
On 15 April 2015, TVB and producer Jazz Boon announced the development of "A Fist Within Four Walls" after casting martial arts star and action choreographer, Philip Ng. Ng negotiated with TVB for three months before agreeing to star. The series would revolve around a group of martial artists living in the Kowloon Walled City, an ungoverned former enclave in Hong Kong. Him Law and Sammy Sum were subsequently announced to star, with Law attached to a villain role. Benjamin Yuen and Moon Lau joined the cast a week later, but Law and Sum dropped out and was replaced by Ruco Chan and Benjamin Yuen, respectively. The cast was finalised in July 2015 with the addition of Ruco Chan, Nancy Wu, Yuen Qiu, and Grace Wong. Philip Kwok was hired as the series action director and choreographer.