Boot Camp

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Finish all the missions in Chapter 1 in Story Mode.

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This requires winning on the Prologue and Chapters 1-3 in Story mode. You can choose Easy difficulty if you find a map challenging, and the game comes with a rewind feature tied to the LB button so if you make a mistake you don't have to completely start over.

You don't need a particular score or even to keep all the orbs in the core or on the map, merely to end the game with at least one orb still on the map. The developer has stated that it is possible to beat each mission in more than 1 million ways, and having played it enough I would tend to agree with that statement.

If you really get stuck ensure that you actively choose your tower items as you play each map as they can dramatically increase your combat abilities. The items you unlock seem to be somewhat random, but all of them result in more effective towers.