When Fortress Gamemode is active, the camera rotation angle is controlled by the game, and the mouse pointer is released.
X camera rotation depends on the height of your current position above the ground: when you are on the ground, the X rotation is 0, when you are 30 blocks above the ground, the X rotation is 90 degrees.
The released mouse pointer can be used to select blocks to build and destroy. Left-click is used to select areas to destroy, right-click is used to select areas to build.
When a world is generated, you spawn with the three pawns around you. You immediately can fly above the ground and give instructions to your pawns.
Sometimes, after the world creation, the camera is stuck with a zero X Rotation angle. To solve the issue, exit the world and then join it again. The camera will start to behave normally.