Success on the battlefield translates to experience points that raise your commander's level, which in turn grants points that can be divided to your liking among four core attributes. In other words, it's as much about building up your commander, unlocking abilities, and purchasing new tower types outside of each level as it is upgrading units within a level. Upgrades extend beyond individual units, applying to experience and credits earned for new towers and power-ups outside each level. Where Sentinel 3 excels is through an array of customisation options that enable you to acquire new units and special abilities. It's a setup inherited from the previous instalment, and a fine one at that. Laser turrets, bombs, missile towers, booster units, and other towers are to be placed tactically so that alien aggressors fry well before they have a chance to attack. You're encouraged to keep barriers intact in the early portion of the level by building an array of defences to prevent enemies from even reaching the wall around your base. This means preserving the gate in front of your base, as well as others situated at various points on the causeway leading to your headquarters. In Sentinel 3, your base must be held at all costs - should a single enemy break through the surrounding protective barrier, it's Game Over. The object, as with all tower defence titles, is to protect a base from aliens that rush across the screen. While there are minor gripes worth calling to attention, on the whole the experience of playing Sentinel 3 is memorable. It's a compelling mixture of depth and immediate action, tactics and strategy, fun and challenging gameplay.

Rather than trying to distinguish itself with odd tactics, unusual mechanics, or features designed purely for the sake of uniqueness, this is classic tower defence garnished with a generous dollop of role-playing. Sentinel 3: Homeworld doesn't reinvent tower defence, but it does do it better than nearly every game before it.

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