It's the history of warfare. Defenders invented moats, fortresses, gates, etc. while attackers invented trebuchets, battering rams, etc. In 21st century defend

It's the history of warfare. Defenders invented moats, fortresses, gates, etc. while attackers invented trebuchets, battering rams, etc. In 21st century defenders have more tools (drones, etc.) to keep the invaders at bay, hopefully.