Below is a list of the rewards you get for completing the levels:
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Beat Level 1: get level 2.

Beat level 2: get level 3.

Beat level 3: get Dual Purpose Rockets and level 4.

Beat level 4: get Seeker Missiles, Light Armor, and level 5.

Beat level 5: get Smoke Screen and level 6.

Beat level 6: get Laser Cannon, Shield, and level 7.

Beat level 7: get Gatling Gun and level 8.

Beat level 8: get Missile and level 9.

Beat level 9: get Rockets and level 10.

Beat level 10: get Cluster Missiles, better Shileds, and level 11.

Beat level 11: get Firebolt Missile, Oil Dispenser, and level 12.

Beat level 12: get better Cannon and level 13.

Beat level 13: get Laser weapon, Heavy Armor, and level 14.

Beat level 14: get better Cluster Missiles and level 15.

Beat level 15: get Anti-Armor Rockets and level 16.

Beat level 16: get best Cannon and level 17.

Beat level 17: get best Cluster Missiles, Self-Repairing Armor, and level 18.

Beat level 18: you've beaten the game.
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