Hills are wildernesses (wilds) that increase your stone resource production if you conquer and own them.
The bonus for owning hills is as follows:
Hills level 1: 5% bonus to stone production
Hills level 2: 10% bonus to stone production
Hills level 3: 15% bonus to stone production
Hills level 4: 20% bonus to stone production
Hills level 5: 25% bonus to stone production
Hills level 6: 30% bonus to stone production
Hills level 7: 35% bonus to stone production
Hills level 8: 40% bonus to stone production
Hills level 9: 45% bonus to stone production
Hills level 10: 50% bonus to stone production
As with all wilds, hills 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.