As the description of this one suggests, to unlock this achievement you’ll need to capture at least 100 fortifications. If you’re wondering what counts as a fortification, it’s essentially any castle or town that you need to besiege to capture. To capture 100 of these might sound like a lot, but it should come fairly naturally as you grow in power throughout your campaign. You’ll need to make sure that you have enough supplies to last in holding a siege, and enough resources to construct appropriate siege equipment.