The Challenger typically requires a Level 6 Tech Center to Unlock, however this requirement may be bypassed if the Challenger is Unlocked via the first Call to Arms Mission.
The Challenger, if Unlocked through the Call to Arms mission prior to having a Level 6 Tech Center :
May be produced in the War Factory and used by the Player at Level 1.
May NOT be upgraded until the Tech Center reaches a minimum of Level 6.
Challengers are heavily armored and fire high explosive armor piercing rounds with a 128 mm main gun. Perfect for dealing some serious damage to tanks
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— In-Game Description
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TOTAL WARFARE - Your units now have complete access to advanced technology.
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— Level 5 Upgrade Message
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MECHANICAL ARMADILLO - Nothing like a little heavy metal, Commander, to get the blood pumping. The Challenger spells murder on the battlefield.
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— Level 10 Upgrade Message
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RULE THE BATTLEFIELD - Your Veteran Level 5 Challenger is your best bet for quick and easy.
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— Level 15 Message [src]
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The addition of an auto-loader improves the rate of fire on the challenger, turning the heavily armed tank into a far more effective killing tool.
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— Level 10 description
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MISSION - Beautiful, Commander. I'm confident with the addition of these vehicles to our arsenal, we'll be able to accomplish any task.
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— Mission - Thicker Skin - Complete
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MISSION - Warfare is out there, Commander. Will you be ready to handle it? Complete these Call to Arms Missions to get a leg up on the competition and earn powerful new weaponry. Start out by destroying a Level 10 Rogue Faction base on the World Map to secure the Challenger tank. Tanks are the core of any ground arsenal. (Note: If you do not already meet the requirements to upgrade this unit, you will still be able to produce it).
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— Call to Arms Mission #1
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MISSION - Well done, Commander! The Challenger is yours, but this is just the beginning. Answer more Call to Arms to earn rewards that are even more powerful.
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— Call to Arms Mission #1 - Complete
Gallery
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Game Update: Apr 17th, 2014 - Call to Arms Introduction
Unlock Requirements
Level 10 Repair Info ( with Level 10 War Factory )