Hills increase the base yield of a tile by +1 Production. They also...
- ...confer a +3 Combat Strength bonus to units defending in them.
- ...provide a vantage point for ranged units.
- ...increase the defense of any city founded on their tile, making it harder for an enemy to capture it.
Below is a list of all the Hills variants and their yields:
Second, a Mine can always be constructed on a Hill even without it containing any resources. The only drawback is that they restrict farmland placement, but Civil Engineering helps to circumvent this limitation for Hills in Plains and Grassland.
Third, Hills provide defensive ground in the event of combat. Ranged units on Hills enjoy both this defensive advantage and a vantage point from which they can shoot at anything in their full range (unless there are other Hills with Woods or Rainforest on them)!
However, it is generally true that a city in hilly land will have trouble feeding its population, especially in the beginning. The Sheep bonus resource, however, helps splendidly with this, since it gives bonus Food, and it's found exclusively on Hills.
Even less appealing than flat desert, about the only thing desert hills are good for is defending in a battle. Unfortunately, the Bedouins, Tuaregs, Berbers, and other desert tribesmen tended to just charge wildly down them.
One way to protect all those riches was to put troops on top of hills when attacked, for (until bullets and bombs were invented) holding the high ground was a pretty good strategy. Thus, in time, hills were dotted by civilization's forts and castles, with fine views over the surrounding countryside.
Hills in plains tend to be landmarks ... just think of the highest point in Iowa, Hawkeye Point, at a stunning 1670 feet (510 meters). Given the flatness all about, these do give nice lines of sight for long distances over the endless, boring vistas.
Very picturesque, very invigorating, beloved of skiers and ski jumpers and other fools. Snow on hills is very nice, and a lot of fun ... unless it never melts.
Hills in tundra lands tend to be barren lumps, for the annual melt carries away the soil and seedlings. However, through history, most towns built in the tundra were put on hills ... which helped keep the wolves and bears out of the streets.