Terrain
The ground decides how a battle plays out: higher ground adds power and sight, water and boulders block the way, and armor and cover make animals tougher. This is what every kind of terrain does.
How much a hit costs: power, +1 from higher ground or −1 uphill, minus the armor of the target (dug-in Turtles get +2) and the cover of its tile. Whatever the sum, a hit that lands always costs at least 1 HP — armor, cover and height make big animals tougher, but nothing is untouchable. Example: a Turtle's bite (power 1) on a dug-in Turtle (armor 1+2) standing on a hill: 1 − 1 (uphill) − 3 = −3, so it costs the minimum of 1. The attack preview shows this sum. Higher ground also sees 1 tile farther per level and over lower obstacles.
Flyers (Crow, Owl, Sparrow and others) jump over water, mountains and rubble, but they must land on a free tile that is neither water nor boulders. Once landed they are ordinary targets: terrain cover and height count, forests do not hide them, and every attack that fits can hit them. Their attacks are sorties: the flyer flies out, strikes and returns to the tile it started on, within one action. Only Bees can shoot at a flyer while it is in the air (flak).
Grass
Open ground: no effect.
Sand
Open ground: no effect.
Forest
Blocks the view, gives cover, and hides animals from enemies farther away in fog games.
Water
Only swimmers can enter; others standing in it get soaked.
Hill
Raised: attacking down from it is stronger, attacking up is weaker, and it sees farther.
High hill
The second level of hills, below the mountains: everybody can walk on it. Raised even higher than a hill: attacking down from it is stronger, and it sees farther.
Wooded hill
Raised and wooded: like a hill (attacking down from it is stronger, it sees farther) and like a forest (blocks the view, hides animals in fog games). It gives no cover on top of that: one strength, not two.
Rocky hill
Boulders on a hill: raised like a hill, but nobody can stand on them or cross them. A smasher (Elephant, Gorilla, Rhino) next to them can break them: a plain hill is left.
Wooded high hill
A wooded hill one level higher: like a high hill (the second level, attacking down from it is stronger, it sees farther) and like a forest (blocks the view, hides animals in fog games). It gives no cover on top of that.
Rocky high hill
Boulders on a high hill: raised like a high hill, but nobody can stand on them or cross them. A smasher (Elephant, Gorilla, Rhino) next to them can break them: a plain high hill is left.
Mountain
The highest ground; only climbers (Chimpanzee) can stand on it. Blocks the view for others; big bonus for attacking down.
Rocky mountain
Boulders on a mountain top: very high and they block the view, and nobody can stand on them. A smasher (Elephant, Gorilla, Rhino) next to them can break them: the mountain is left.
Swamp
A unit that stands here can only move 1 tile.
Rubble
Boulders: nobody can stand on them or walk through them, and flyers cannot land on them. Elephant, Gorilla and Rhino can smash one on a tile next to them: it turns into plain ground.
Ice
Frozen water: everybody can walk on it, swimmers included - but it is slippery: a walk that ends on an ice tile does not stop there: the animal slides one more tile in the direction it came from (if that tile is free; otherwise it stops on the ice). Flyers do not slide. Heavy animals (Elephant, Gorilla, Rhino and the like) can cross it too, but it breaks: the ice they walk away from turns into water, and if they stay on it, it gives way at the start of their next turn - then they sit in the water (1 step to climb out, 1 damage per turn until they do). Flyers can land. A grenade or boulder (Chimpanzee, Macaque, Orangutan, Bear, towers; a flyer's bomb) thrown on it breaks it into open water, also under an animal it hits. It melts in a heatwave.
Fountain
A fountain of youth: an animal that starts its turn standing here gets 2 hit points back (flyers in the air do not).