castle
城 centers on a fortified stronghold: a castle, typically built with earthworks and walls.
城 combines 土 (earth) with 成, which likely contributes the sound and the idea of completion, suggesting earthworks that form a completed fortification.
The left side is earth 土, and the right side 成 suggests something built up. Picture earth piled high to form the walls of a castle.
For ジョウ, imagine a grand castle with a showy gate: show -> ジョウ, and the earth walls make it a fortress.
castle
Ibaraki (prefecture)
Imperial Palace (esp. from 1888 to 1946)
Miyagi (prefecture)
castle wall; town walls; defensive wall; rampart
Yamashiro (former province located in the south of present-day Kyoto Prefecture)
mountain castle
impregnable castle
lord of a castle
castle town; city in Japan that developed around the castle of a feudal lord
castle town
long (defensive) wall
fortress; stronghold; citadel
castle site; ruins of a castle
castle gate
old castle; old fortress
castle construction; building a castle; fortification
inside of a castle
Edo Castle (residence of the Shogun during the Edo period, now the site of the Tokyo Imperial Palace)
land near a castle