The noun il muro has two correct plurals: i muri and le mura. This is an example of plurale doppio, i.e. double-plural.
They are used in two different contexts.The feminine version of the noun is used the most often, and refers to “walls” in the sense of the entity that is around/supporting something. It also the preferred noun when referring to historical structures:
Le antiche mura etrusche (The ancient Etruscan walls)
Siamo davanti alle mura del centro storico (We’re standing in front of the walls around the old town centre)
Le mura di casa (The house’s walls)
The masculine version is used when referring to walls in the structural sense of the word, for example:
I muri portanti (The supporting walls)
I muri doppi (The double-walls)