Fortified Cave With Cache

This is the upgraded form of Fortified Cave, available with Pottery technology.

This tile gets some defensive bonuses like: This building can be bombarded by an enemy unit, lowering its defensive bonus.
 * +90% air defense.
 * +25% tile defense.
 * City Garrison upgrade has effect in this tile.
 * +1 visibility height.

Besides, it add 1 in the tile where it is located, plus influences the borders around it, up to a distance of 1.

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

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.