Follow the stud trails until you reach Weathertop to enter a cutscene.
You need 3 firewood for the bonfire. First is located in the background, smash the plant to get it. Second is in the foreground, destroy the gray plant to get it. Lastly, you need to go up the ramp on the upper left. Hit the rock, then press Circle to build the sliding floor below. Go back down and push the boulder, then enter the small hatch. Hit the rock on the left to get the last firewood. Build fire on the bonfire area below.
Enter the small hatch again, this time jump up to the bar on the right of the ledge, then hit the bird's nest and pick up the egg that will drop below. Use Sam to grow the tomato plant on the lower part area. Hit the tomato plant and pick up the tomato. Use Merry to catch a fish in the fishing pond near the ramp. Give all the ingredients to Sam to have them cooked, use Sam to cook all the food in the bonfire.
Destroy all objects on the left and build a bridge. Cross the bridge. Switch to Sam. Grow a plant, then pick up rocks and throw them at two vines on the left. Jump across. Use Sam again to grow another plant, jump up the plant to the ledge above. Crawl under the small hatch. Build the bonfire, light it up and burn the vines.
Follow the path upwards. Jump up and hang onto the vines to bring them down. Press Circle to build a bridge then cross over it. Continue hiking upwards. Use Sam to destroy the plant below the climbable area. Build the bonfire and light it up. Grab the bars and continue pulling up. Use Sam to grow yet another plant and jump across it. Exit on the door at the trail above.
There are 5 bonfires you need to lit up. First, Use Sam to light up the one in the center area. Second, grow up a plant on the left side, then smash it and assemble the parts, then light it up. Third, smash the wagon in the background and assemble the parts, then burn it. Fourth, you need to do this with Frodo, near the center area assemble some white-colored lego parts, then switch to Sam to burn it. Finally, on the top left area, push a statue using Sam. Then switch to Frodo to repair it, then again to Sam to light it up.
Here you need to hit 5 ringwraiths with the torch. Hit the very first one with Aragorn's torch. Second, use Sam to build a bonfire on the left area close to where the wagon was before. Light up the torch and hit another one. Next, there are 3 rocks on the grounds. Pick one, throw it at any of the vines in the side areas, then pick up the torch that drops to the ground. Light up and fire away. Repeat this for the last few enemies.
As Arwen, simply follow the stud trails on the next two sequences to trigger cutscenes. Then as Frodo, continue following the studs again until you reach the council.
Switch to Legolas, follow the trail, use his bow by holding Square and aim at the rocks on the upper left side of the river. Shoot the plants 3 times to make the rocks fall down. Cross the river and follow the path again.