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In multiplayer civilization it´s very important to be aware of the vision range of units and culture in order to be able to build up a working sentry net against possible attacks and on the other hand to know when an opponent would see your own stack moving around or towards his land. Knowledge of vision ranges is a basic condition for the planning of any efficient attack. The less turns your opponent knows exactly (or at all) what you are going to do, the better - often being seen as late as possible is the only way of being able to capture a city. The same is true for defense, the more turns you know in advance what exactly is coming to you and where, the more you will be able to prepare.
Vision Range Mechanics
land units
* Standard vision range of land units is 1 (picture 1).
* The sentry promotion (available after Flanking I), mostly used on Horseback Units, increases vision range by 1 (picture 2).
Rule: Land units have a base vision range of 1, it increases to 2 with the sentry promotion.
* Standard vision range of cultural borders is 1.
* Hills on cultural borders have a vision range of 2.
* Peaks on cultural borders have a vision range of 2 and see above forests and hills as if those were not on the tile.
sea units
* Standard vision range of sea units is 1 (picture 3).
* The technology Optics increases vision range of sea units by 1 (picture 3).
* The sentry promotion increases vision range of sea units by 1 (picture 3).
Rule: Sea units see land one tile further away then they usually could look.
Rule: Sea units don´t see a hill/peak "stand out".
* A hill increases the vision range of a unit standing on it by 1 (picture 4).
* Forest, Oasis, Hills and Peaks decrease vision range by 1 (picture 4a).
* Hills and Peaks stand out, meaning a a unit sees them if it was otherwise 1 tile short in vision range, unless the tile is more then 3 tiles away (picture 5 and 6).
* Units look over the "first" water tile as if wasn´t there (picture 7 and 8).
Rule: No unit ever sees further then 3 tiles!
In an ancient start 5v5 teamer you can very early in the game build up a sentry network with 3-4 units that allows you to expand without cover for your cities without having to fear an attack by for example 5 chariots. Once your sentries spot those, you just slave out 2-3 spears and are totally safe without having given up early production unnecessarily or too early before you could use for workers/settlera and still have the perfect amount of defense against what´s thrown at you.