old castle; old fortress
Refers to a castle or fortress from a past era, often one that is now a historical site or ruin.
Several medieval old castles remain in this region.
We stayed at a hotel converted from an old castle.
An old castle stands atop the mountain.
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').