inquebrantable adj mf | formal (que no se rompe) | unbreakable adj |
Los cascos están hechos de un material inquebrantable. | ||
The houses are made of unbreakable material. |
While one of the definitions of 'casco' is 'farmhouse', it seems that 'casco' in this sentence is a helmet.
Therefore, I suggest that the sentence should read as "The helmets are made of unbreakable material."