old castle; old fortress
Refers to a castle or fortress from a past era, often one that is now a historical site or ruin.
この地域には中世の古城がいくつか残っている。
Several medieval old castles remain in this region.
An old castle stands atop the mountain.
We stayed at a hotel converted from an old castle.
I visited an old castle in Europe during my trip.
The remains of an old castle still stand on this hill.
古城 refers to an old castle that may still stand in some form, whereas 城跡 refers only to the remaining ruins or former site of a castle.
Compound of 古 (こ, 'old') and 城 (じょう, 'castle').