The character 城 was pronounced "jiyau (jou in modern kana usage (as laid out by the Japanese government in 1946))" and the character 柵 was pronounced "saku" later.
後に、それぞれの字は、城を“じやう（＝現代仮名遣い：じょう）”と読み、柵を“さく”と読むようになった。