"Before the real battle begins, you will have to reach the wall itself. Choose Legolas and shoot the target on the left side of the grate and build a mechanism from the received cogs (K). Afterwards use Gimli to break the cracked brick on the ground, pick up the crank and connect it with the rest of the machine. After attaching the crank, use the broken chests to build a platform and use it to get onto the wall. The last obstacle can be once again destroyed by Gimli.
During the last wave you will have to throw Gimli at the cracked brick and shoot down two Orcs with Legolas' bow. Unfortunately the wall will be broken and you will an up by its base.
Use Gimli's help to break the cracked brick found on the wall. Build a platform from the received elements, thanks to which you will be able to reach the upper one. Destroy the spears by the wall and build a target from their remains. Time for Legolas: place an arrow in the hole and leaves making it possible for you to reach the bar will appear. Swing on it, jump to the left and push down the boulder which will let you the team reach the top of the wall. Break the cracked brick with Gimli and you will be able to head further to the left, inside the fortress.
You need to give the defenders inside the fortress some time to prepare for the battle. Pick up Gimli and throw him at the broken Uruk-hai shield. Aragorn will be afterwards able to join the fight by jumping across the cracked floor. Defeat the enemies, destroy the ram and reinforce the gate with the LEGO bricks on the ground.
The action will move to the wall above the gate. Controlling Legolas, you will have to show down three ladders - a fourth, bigger one will appear once you do. Use the elf's daggers to destroy both hooks and unhook it from the wall. Use the remaining pieces to build a floor and push down the chest. There, after switching to Aragorn or Gimli you have to build a platform and use it the reach the rope which can be used to climb the castle.
The gate has fallen - time to face Saruman's forces. You control King Theoden and you have to kill as many Uruk-hai as possible while collecting studs as well. You can also admire the views:) It's the last stage of Helm's Deep defense.
This time you need to reach the gates of Isengard - Saruman's seat. On your you can buy missing characters (I'd recommend the Berserker who you will be able to use mithril bricks and gain more treasures). Unfortunately, after the battle of Helm's Deep, Saruman's gaze will be pointed here and therefore blocks your path. However every cloud has a silver lining - thanks to this you can switch to the group of Hobbits and enter Osgiliath, which you should do now.