The tunnel is a defense structure of the GDI in Tiberian Twilight.
Background
An instant-transport network, a Tunnel system allows Commanders to get their forces from Point A to Point B at a moment's notice. Upgrading adds shields and ups capacity to include heavy units.
In-game
The tunnel is the second unlock in the Defense class. Relatively cheap, costing 2 power points to make one. It has a weaponless counterpart, the Tunnel network. Upgrades include a shield and allowance for heavy Tier 3 units to enter and exit the tunnel. Enhanced shields can provide cover to the tunnel a bit further.
 Global Defense Initiative Fourth Tiberium War Arsenal | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
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