Switch to Wicket and go right to crawl through the tunnel. Repair the bridge so that the rest can join you. Go further and destroy the planks that are blocking the entrance to the shack that contains a 3PO panel. Use it and build a grapple point. Go further to the right and down the stairs.
When you reach the chessboard, destroy everything that is on it and build a crate of the rubble. Push it towards the handle to the left. Now as Wicket use the nearby tunnel and use the lever. Go down the screen and stand on the buttons. When all your characters are in place, the elevator will go down. Build a turnstile on the right and use it to enable Wicket to enter the tunnel. When he's up, stand on the button. The passage is open. Go right and cross the river. Shoot the mechanism between the two wooden gates. Fix a panel and use it as 3PO.
Stand on one of the buttons and the rest of the characters will join you. Now approach the right gate and use the planks to make a footbridge for the droids. Near the left gate use the tunnel as Wicket and pull the lever. Build a grapple point. Now stand on one of the buttons and the second pair of gates will be open.
Use the catapult - destroy the gate and shoot AT-ST. Take over the walker and go forth to deal with all the enemies. When the battle is over use AT-ST to reach the button in the right side of the clearing - that will let some other character use the lever. Build the grapple point and use the hook twice to reach the bunker's roof. Destroy the devices there and complete a track for the crate. Push the crate down and use its remains to construct a panel for R2. Also, build a footbridge for the droid from the planks nearby. Enter the bunker.
Blast through the door with flashing, red diodes. Use the blue lever to the left and you'll disable the first lock. Now destroy the door to the left and go to the next room. Defeat the stormtroopers and find the three buttons on the floor. Jump up the stairs and push the crate into its station. Now you can use 3PO to go up there and use the panel - it will give you access to a lever that will turn off the second lock.
Go back to the previous room and this time go right. Use R2's panel and use the elevator. Build a little bridge out of the bricks there and as 3PO use another panel. Pull the lever and the third lock will be disabled. Go to the left end of the upper platform and as R2 hover across even further, along the glass wall. Use the panel and form a tunnel for Wicket of the loose bricks. Use it to get to the other side, where you'll find the control lever for the last lock. Get back to the central room and enter the power supply room to build 4 bombs in there.