moenia

Moenia is a plural-only noun - there is no singular in use - and it declines as a third declension neuter i-stem: moenia, moenium, moenibus. Adjectives must therefore be neuter plural and the verb must be plural, however singular the English translation sounds.

The distinction from murus is real and useful. A murus is a wall as a piece of masonry; moenia are the whole defensive circuit of a city, walls, towers and gates together, and by extension the city as a fortified thing. Virgil's moenia Romae is the walls and the city in one breath. The word belongs with munio, "to fortify", and munimentum; the near-identical munia, "duties", is a different word that shares only the look. English munition and muniment are the cousins that survived.