Muro vs. Mura
In Italian, both muro and mura can be translated as wall, but they are not interchangeable. Their usage is distinct and depends on the specific type, scale, and function of the wall being described.
Muro
A1Muro is a masculine singular noun (its plural is muri) and is the general, common term for a wall. It typically refers to a single structure, such as a wall inside or outside a house, a garden wall, or any standalone dividing structure. It can also be used figuratively.
Ho appeso un nuovo quadro al muro del salotto.
(I hung a new painting on the living room wall.)
Il gatto cammina sempre sul muro che divide i nostri giardini.
(The cat always walks on the wall that divides our gardens.)
I muri di questa casa sono molto spessi e isolano bene dal rumore.
(The walls of this house are very thick and provide good insulation from noise.)
Dopo la discussione, ha eretto un muro di silenzio tra di noi.
(After the argument, he erected a wall of silence between us.)
L'artista ha dipinto un murale su un grande muro in centro città.
(The artist painted a mural on a large wall in the city center.)
Mura
B1Mura is a feminine plural noun that specifically refers to the large, defensive walls that encircle an entire city, town, or fortress. It denotes a complete, large-scale fortification. It is also used in the expression tra le mura domestiche, meaning within the home.
Le antiche mura di Lucca sono famose in tutto il mondo.
(The ancient walls of Lucca are famous all over the world.)
In passato, le mura proteggevano la città dagli attacchi nemici.
(In the past, the walls protected the city from enemy attacks.)
È possibile fare una passeggiata panoramica lungo le mura di Ferrara.
(It is possible to take a panoramic walk along the walls of Ferrara.)
Molti segreti si nascondono tra le mura di questo vecchio castello.
(Many secrets are hidden within the walls of this old castle.)
È una persona riservata che preferisce stare tra le mura domestiche.
(He is a private person who prefers to stay within the walls of his home.)
Summary
In essence, use muro (plural muri) for a standard, individual wall of a building, room, or garden. Use mura (which is always plural) when referring to the large, encompassing defensive walls of a city or fortress.