Sentinel 3: Homeworld doesn't reinvent tower defence, but it does do it better than nearly every game before it.

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.

It's a compelling mixture of depth and immediate action, tactics and strategy, fun and challenging gameplay.

While there are minor gripes worth calling to attention, on the whole the experience of playing Sentinel 3 is memorable.

Make E.T. phone home

The object, as with all tower defence titles, is to protect a base from aliens that rush across the screen. 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.

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.

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. 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.

It's a setup inherited from the previous instalment, and a fine one at that. Where Sentinel 3 excels is through an array of customisation options that enable you to acquire new units and special abilities.

Role-playing disguised as tower defence

Upgrades extend beyond individual units, applying to experience and credits earned for new towers and power-ups outside each level. 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.

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 addition, commander abilities - repair drones, rage mode to boost attack power, and jet jump to hop around the map to name three - unlock at certain levels, giving you an advantage during tough battles.

Even more, you can acquire new towers and special weapons such as homing missiles and the destructive ion bomb by spending credits. Buying things isn't a new concept, but requiring the purchase of tower and weapon slots is a unique development.

Edge off the excellence

Unfortunately, it's also one of the game's few weaknesses. You start with a small number of tower and weapon slots that only expand when you spend credits.

The developer's goal in making you pay for slots is to force you to strategically consider which towers and weapons you want for the next level. It's understandable, though unnecessary: it feels like one more thing to buy when there are already plenty.

Also of concern is the difficulty, which inclines steeply. It's hard to escape the sense that the game is tougher than necessary, yet multiple levels of difficulty counter this complaint - just don't be surprised when you have to bump it down midway through the lengthy campaign.

An expanded selection of modes - a campaign joined by Endless levels and scenario-driven Classic stages - contributes to the game's greater value and variety. Of course, robust integration of leaderboards and achievements via OpenFeint and Game Center furthers replay incentive.

By pairing a full list of features with basic role-playing and strong tower defence gameplay, Sentinel 3 offers a rare depth and sense of investment that builds between levels rather than solely within each stage.

Instead of grasping at risky new gameplay ideas, this is a game that establishes a new standard of quality in a crowded genre with polished gameplay that's fun and involving.

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