固

Outer enclosure radical - the big square wraps everything in, written as the first two strokes (top-left descender and top + right side) and closed on the last stroke. The indexing radical. The bigger 囗 is distinct from the smaller 口 (mouth) it visually resembles. Same family: 国 (country), 园 (garden), 困 (trapped), 围 (surround).

Inside 古 supplies the sound - gǔ drifting to gù through tone shift. Helpfully, 古 means 'old, ancient' - so 固 reads as 'enclosed and old,' i.e. firmly established by time. The phonetic doubles as a semantic clue for the 'firm, solid, sure, established' meanings. Sealed walls plus age give a picture of unshakeable stability.