Buildings
Besides animals and terrain, the board holds structures: walls and palisades that block, bridges that connect, pits that trap, and in production games headquarters, factories and mines. Here is what each of them does.
What you can find and build on the board. Beavers build most of them (and only in production games the headquarters, factory, palisade and nest animals); the limits are per team.
Headquarters
12 hit points. Every team in a production game has one next to its army; new animals up to price 4 come out of it. It blocks its tile like a wall and only enemies can shoot it. If it is destroyed, the team is out together with its animals and orders. In fog it sees 3 tiles.
Factory
A Beaver builds it on a free grass, sand or hill tile next to it for 4 diamonds (production games). 8 hit points, at most 2 per team, only enemies can shoot it. It makes the heavy animals (price 5 to 8) that the headquarters cannot. It does not keep a team alive. In fog it sees 2 tiles.
Wall
A Beaver builds it on a free tile next to it where a walker could stand (6 hit points, at most 10 per team). Nobody walks or slides through it and a flyer cannot land on it; shots and sight pass over it and ground jumpers jump over it. A pushed animal is squashed against it. Anybody may shoot a wall, even their own to get out.
Thorn bush
A Beaver plants it on grass, sand, swamp or hill (1 hit point, at most 2 per team). It blocks walking like a wall, everybody can see it. At the end of a team's turn every animal of that team within 1 tile of an enemy's bush is stung: 1 damage and poison for 1 turn. Your own bushes never sting you.
Pit
A Beaver digs it on grass or sand (1 hit point, at most 3 per team). It is hidden from the enemy in fog games. The first enemy that walks or slides over it (flyers pass) takes 1 damage, stops there and cannot move for 2 turns (heavy animals: 3). The pit is used up. Your own animals do not trigger it.
Bridge
Where a map had a bridge and it fell, a Beaver next to the water tile rebuilds it: the work takes 3 turns and the Beaver is busy meanwhile. 4 hit points. Everybody can walk over it. Only the heavy artillery (Eagle, Bear, Coconut Tower) can destroy it, and only while nobody stands on it; a hit on an animal on the bridge also damages it by 1.
Palisade
Production games: a Beaver next to a mine of its team builds a wooden wall round the flag in 3 turns (4 hit points, one per mine). Once it stands, nobody can step onto the flag tile. While it stands, enemies cannot work on taking the mine. Enemies can shoot it down like a wall.
Lookout
A Beaver builds it on a free tile next to it (4 hit points, at most 5 per team) in 1 turn. It sees 4 tiles (6 on a hill) in fog games and reveals camouflaged enemies in that range.
Gold mine
Yellow flag. Gives +1 gold per turn to the team that owns it. Stand alone within 1 tile of the flag: a free mine is yours after 2 turns in a row, one of an enemy after 3.
Diamond mine
Blue flag. Gives +1 diamond per turn to its owner; diamonds pay for factories, heavy animals and nest animals. Taken like a gold mine.