Fortified Cave

Once a civilization discovers Shelter Building, the player can use a worker unit to fortify a cave by sending this worker to a tile (not necessarily inside the player's cultural border) and pressing the command button build Fortified Cave.

This tile gets some defensive bonuses like: This building can be bombarded by an enemy unit, lowering its defensive bonus.
 * +50% air defense.
 * +15% tile defense.
 * City Garrison upgrade has effect in this tile.
 * +1 visibility height.

When a unit stands on this tile for 17 turns, and all the requirements are satisfied, it is automatically upgraded to Fortified Cave With Cache.

Strategy
Build this improvement on strategic tiles outside a city's economic range; for example to block the movement of (future) enemy units into a certain territory, or on resources as it will eventually become a fort and then will provide this resource to your connected cities.