筑

Bamboo-top radical — the compressed-top form of 竹, the indexing radical, picturing two stems of bamboo with their leaves. Ancient construction used bamboo scaffolding and bamboo-frame moulds to ram earthen walls; the radical anchors 筑 in the building/material family while pointing to the bamboo tools of the trade. Same radical caps 笔 (brush), 箱 (box), 简 (slip).

Bottom 巩 supplies the sound (gǒng → zhù via Old Chinese phonological drift) and a strong semantic bonus: 巩 itself means 'to bind tight, consolidate' — exactly what builders did when ramming earth between bamboo boards to make a wall. The phonetic and the meaning reinforce each other almost perfectly. Same character heads 巩固 (consolidate).