An old castle is still an old castle. If it was an old castle, that implies it is something else now but formerly it was an old castle. It was an old castle that we just saw OR what we are looking at not was an old castle but no longer is'
Serious ambiguity in the question. Either answer could be correct depending on context
How would you say: ''It was an ancient castle.''C'était un château ancien.C'était un ancien château.