Mountains are wildernesses (wilds) that increase your iron resource production if you conquer and own them.
The bonus for owning a mountain is as follows:
Mountain level 1: 5% bonus to iron production
Mountain level 2: 10% bonus to iron production
Mountain level 3: 15% bonus to iron production
Mountain level 4: 20% bonus to iron production
Mountain level 5: 25% bonus to iron production
Mountain level 6: 30% bonus to iron production
Mountain level 7: 35% bonus to iron production
Mountain level 8: 40% bonus to iron production
Mountain level 9: 45% bonus to iron production
Mountain level 10: 50% bonus to iron production
As with all wilds, Mountains have defenses that increase in strength according to the level. The number of defending troops per level is the same across all the different types of wilds.