矗 is three 直 (straight) stacked up: triple-straight means standing perfectly upright and tall. The everyday word is 矗立 (to tower), for monuments, towers, and mountains.
The top 直, picturing an eye sighting along a straight line. Indexed under Kangxi radical 109 目 (mù, eye), which sits inside this 直. Stacking three of them piles 'straight' on 'straight' to mean towering.