窗

Top 穴 — the cave/hollow radical, the indexing radical for 窗. Pictures a cave entrance or roofed pit dwelling. Anchors 窗 in the architectural-opening family alongside 空 (empty), 突 (sudden / abrupt opening), 穿 (pierce through). A window is, in essence, a deliberate hole made through a wall.

Bottom 囱 — supplies the sound (cōng → chuāng, retroflexion shift) and reinforces meaning: 囱 itself is an old pictograph of a smoke vent in a roof. So 窗 layers 'opening under cover' (穴) over 'smoke-vent' (囱), giving the literal hole-in-the-wall picture of a window.