Complex sentence: Cofiwn's last major action was their response to the "Wales Festival of Castles" in 1983, which they countered with a program of actions, "Sarhad' 83" (Insult' 83).
Split into simpler sentences: Cofiwn's last major action was its response to the Wales Festival of Castles. Cofiwn countered with a program of actions, "Sarhad' 83" (Insult' 83).