The set is an extension to the Battle of Helm's Deep set and represents the battle between the Rohirrim with Éomer and the Uruk-hai Army.
Éomer is nephew of Théoden, King of Rohan. Éomer works with Aragorn to him the Rohirrim fighting against the onslaught of Uruk-hai orcs.
Of course these characters also appear at other points in the story, but the fortress wall included in the set places them at the Battle of Helm's Deep. The giant crossbow features just after Aragon tosses Gimli ("don't tell the elf") and then the castle is breached.
This scene takes place in The Two Towers, the second volume of the Lord of the Rings trilogy in book and movie form.
Did you like the Lego Uruk-hai Army?