Left-side 土 is the earth radical - walls in pre-modern China were rammed earth or mud-brick, so the material itself indexes the meaning. Same family as 城 (wall/city), 堵 (block), 坡 (slope), 堆 (pile). The radical literally tells you what the wall is made of.
Right-side 啬 supplies the sound (sè → qiáng, drift via the same Old Chinese root). 啬 originally pictured grain stored in a barn - the walls of that barn are the conceptual link to 墙. Same phonetic in 蔷 (qiáng, rose), preserving the qiáng reading.
