故事发生在火星战事之后，外星人仍没有停下侵略的脚步，而这次它们的目标居然是人类的家园——地球，玩家要抵御不断从外星飞船上汹涌而出的异形，充分利用各种先进的防御塔组成防御工事。游戏和前作一样有着精美无比的画面，并新增了战舰技能、能量采集等多种新要素，使得游戏的策略性更强。

主要特点：
-当屏幕右下角黄色的能量条被填充后可释放哨兵的战舰武器，每填满一格都有一种不同的技能
-放出移动攻击的微型机器人能够对敌人进行骚扰，从而提高防御的稳定程度
-建立加速器能够对防御产生正面的影响，每种防御塔升到最高级时都能获得威力极大的技能
-玩家要和大量新型外星怪物作战
-关卡模式中能够考验玩家快速进行战略部署的能力，还能够获得各种称号
-内置的OpenFeint能够提供完整的在线高分榜、称号、好友以及聊天系统
-由Specimen A独家为游戏创作的背景音乐

• Unleash the Sentinel's incredible ship weapons system - if you've got the power to do so...
• Deploy mobile attack drones to harass the enemy and augment static defenses.
• Construct boosters to buff your offense, and upgrade to top-tier towers for devastating special moves!
• Pit yourself against previously unseen members of the star spawned horde.
• Mission mode provides quick tactical challenges. Beat a scenario to earn achievements! 
• Full online high scores, achievements, friends and chat using OpenFeint
• Exclusive new music written by Specimen A
• OS3.0 iPod Music Player (OS3.0 or higher required)

版本 2.0.0 中的新功能

- GameCenter added for leaderboards & achievements. This replaces OpenFeint (removed due to the service stopping)

- iOS 6 compatibility

- We hope you enjoy the Sentinel series as much as we enjoy making them. If you haven't already, check out Sentinel 3: Homeworld and coming in 2013 - Sentinel 4: Dark Star!