After getting the access key to the large cogwheel on Myst, you find the linking book to the Mechanical Age.
Enter the Fortress
From the place where you arrive, walk across the bridge to enter the building there.
Fortress Rotation Simulator
If you go to the right, you enter a room with many weapons, and a throne-like chair. To the left of the chair, there is a device. If you check on it, it shows a Fortress Rotation Simulator. After the introductory sequence, it shows a circular shape with a red needle pointing downward.
If you lift the left lever, it "unlocks" the needle, and if you lift the right lever, the needle will rotate counterclockwise. The needle is drawn to the red triangles around the edge like a magnet, so if you lower the right lever when the needle is near one of the triangles, it will stop near it. You can then lower the left lever to lock the needle in place at that triangle, which will play a sound. There is a different sound for each direction. You might want to write down a description of each, along with the direction it corresponds to.
Red Page and Blue Page
In the room with the Fortress Rotation Simulator, look at the throne. There is a small door to the left of it, which has a yellow stripe on it. Click it to open it. Check on the shelf to find a blue page.
If you go back into the hallway, depending on which direction you go, you will see a red button on the wall. Before pressing it, keep going until you reach a room with paintings on the walls. Look at the throne, and click the wall to the right of it, beneath the tapestry, to open another hidden room. In the treasure chest in the corner, you will find a red page. There is also a letter in the wine rack that you can read if you wish.
Rotate the Fortress
Go to the back of the hallway and press the red button to lower the stairs. Go down and lift the lever, and let go of it when the symbol to the left turns red. Then go back the way you came, then turn around and press the red button to make the stairs rise again.
Walk straight ahead to enter the elevator. Press the up arrow to go to a new room. Then, while still in the elevator, press the rectangular button, but go through the door before the elevator descends. Then when it descends, you can use the device on the top of the elevator to rotate the fortress. You won't be able to see the rotation, so you will have to rely on sound to tell you when the fortress has rotated to a different direction. Lift the left lever to The rotation, lift the right lever to rotate the fortress, then lower the left lever to lock it in place. The sound that plays will tell you which direction the fortress has turned to.
When you have rotated the tower (making sure to lower the left lever to lock it in place), enter the elevator and press the down arrow. Go outside, and if you rotated the tower to a new direction, you can cross the bridge to a new area. To the east, there is a podium that displays two symbols on the right. The symbol on the left is a circle with three triangles below it, and the symbol on the right is the left half of a circle. To the north, there is a podium with two symbols on the left. The symbol on the left is a circle with a notch in the bottom, and the symbol on the right is a triangle pointing down, a rectangle, and a triangle pointing up. You can rotate the fortress to the west, but there is nothing there. However, if you look through the telescope in Sirrus's room while the fortress is pointing west, you will see something interesting.
Return to Myst
Make the fortress point south again, then you can go outside, cross the bridge, and change the symbols on the podium to match what you saw on the other islands, then press the red button. This will make the staircase lower. Walk down it to find the Myst linking book, which you can use to return to Myst Island. But before you go, you can go back to the rooms in the fortress and take the red or the blue page to bring back with you. You can come back again to get the page you didn't get last time, if you wish.