Guard Towers are structures that will spawn guards hostile to attacking forces. You can purchase them from Siege Engineers in NPC Towns, and place them throughout your town to help you defend during a Siege. You can also place them outside your town for provide troop reinforcements in a wild skirmish, or to help you defend your War Camp when you’re sieging an enemy.
Guard Towers can be defeated. Every time a guard is killed, the Guard Tower loses some of its Magicka. When the tower runs out of Magicka, it is destroyed.
A Sentinel is a unique block that you can use to protect your chests and doors. When an attacker attempts to open a chest inside the Sentinel's 16 block range (cube), a powerful Sentinel mob is spawned near the chest, and the attacker must kill the mob to gain access to the chests within the Sentinel's domain. Sentinels are very difficult to defeat alone, so be careful when planning your final push into an enemy's vault - they might have many Sentinels protecting the area!