The terrain spaces surrounding your City are called valleys. Each has a terrain type and a level associated with it(see below). When hovering over a valley with the mouse pointer, you will see information about the benefit you will receive when you have conquered it. Clicking it will give information about current ownership and also give the possibility to scout it out or attack it. Scouting or attacking a valley (without a city!) does not break the Beginner's Protection, even if it is currently owned by another player.
Each valley has one of the following terrain types: Flat, Grasslands, Swamp, Lake, Hill, Desert or Forest. All of these terrain types, except for Flat, provide a percentage production increase to your city's resource production when you have conquered them.
The Flat does not give a resource production bonus when conquered. However, you can build a new city on it, once you have reached the required prerequisites, including:
Once you have conquered the flat, you can build a city on it. Building a city on a higher level flat will not give any bonus, unless it was built to make an NPC out of it by abandoning it after recalling the troops, in which case the level affects the level of the created NPC.
A grassland gives a minor production bonus for Food. Swamps and Lakes give a lot more production, so it is better to go for them, especially lakes.
Swamps offer a higher food production bonus than grasslands, but lakes still give better bonuses, so they should be conquered, rather than swamps.
Lakes are a must for food-cities, as they provide the highest production bonus out of all food-type valleys. That is why it is recommended to conquer as many level 10 lakes from a city as possible.
Hills are the only kind of resource field which give a bonus in Iron production. As with all valleys, they give the highest bonus at level 10.
Deserts give a bonus to the Stone production when conquered. On older servers where stone is not needed as much as it is very cheap in the market, there is no need to conquer them.
Forests give a bonus Lumber production when conquered, and the bonus is highest at level 10. Lumber is a very good resource to have, as it is needed for a lot of building, research, and troop building.
Valley production bonus
The following table gives an overview of terrain types, their levels and the associated resource production bonus:
If you watch a valley over time, you will see that its level goes up slowly until it reaches 10. Everyday the valley will increase 1 level, until it reaches level 10 then it changes back to level 1. Keep track of this if you want valleys of a particular level because if you wait long enough, you will have the level you want, though you may have to wait some days or weeks.