You mean 'muraille' means quite literally a defensive wall right? I thought it could also be a city wall, like in the Chanson de Chartres (our national anthem is based on this melody so I am really familiar with the text
Ou si monsieur le prince
Envoyra son canon
Au pied de voz murailles
But I suppose a 'muraille' is always made of stone, especially for contemporaries in the 11th-16 century. I mean if we think about a defensive wall, we wouldn't be suprised if it is made out of other material than stone anno 2017. However, I guess these Frenchies from Medieval time always meant a wall made out of stone, so in my opinion it is uselss to add the 'of stone' part in 'wall of stone'. It's not like a knight went from the court of his king to a city to deliver a message, only to see a 'muraille' in a poor, wooden shape. These walls were always made of stone, especially around a city.
Odly enough, in the original texts of the Chanson de Roland, I found at least 3 instances of 'mur' were French medieval historian Joseph Bédier translates it to 'muraille' (5 (I), 97 (VIII) and 237 (XVI)). The contexts shows us that in all three cases it was definitely a defensive wall, so I guess Bédier wasn't crazy or something. Of course at the time city walls as we know them were very well known in all of Europe, but apparentely in the language that was used in the Chanson de Roland there was no distinction between a normal wall in a building and a wall that was used in a defensive way.
I believe this distinction was not made in Latin either, but correct me if I am wrong. So muraille appears to be a word that was introduced later, perhaps when the fortications in Europe changed because of the increase in weapons by gunpowder, perhaps earlier but after the Chanson de Roland. But nevertheless, since already modern words like 'Fortresse' are used, 'Muraille' would be perfect for a stone wall in my opinion.
I don't know what the wooden wall would look like in Empires Apart, is it like a palisade? Because then we only need to add a extra 's'