Learn Italian: Il castello era circondato da un muro di pietra. - The castle was surrounded by a stone wall.

In the passive voice, the tense is shown by the verb essere (to be). We use the imperfect era here because it describes an ongoing state or background description in the past. If the wall had just been built around it in a specific, completed event, you would use the passato prossimo: Il castello è stato circondato (The castle was/has been surrounded).

Why do we use da for "by" in this sentence?

In passive sentences, da introduces the "agent"—the person or thing that is actually performing the action. Since the stone wall is doing the surrounding, it is introduced by da to mean "by a wall".

Does circondato have to agree with the subject here?

Yes! In passive sentences built with essere, the past participle acts like an adjective and must agree with the subject in gender and number. Since il castello is masculine singular, it is circondato. If the subject were feminine, like la città (the city), it would be era circondata.

Why is the wall called un muro instead of una parete?

Muro refers to an exterior, heavy structural, or freestanding wall (like a castle wall or a brick wall). Parete is generally used for the interior walls of a room.