Roran used a hammer, originally owned by Horst, as his trademark weapon. He initially used it as he helped the civilians of Carvahall rebel against the Ra'zac and Galbatorix's soldiers and free the village. Roran (partly inspired by Gerand in "The Song of Gerand") continued to use the hammer as his weapon of choice as he led the villagers of Carvahall through The Spine to Narda, Teirm and finally through the Boar's Eye to Surda, earning the nickname "Stronghammer". Also, at the Battle of The Burning Plains, he used it to kill The Twins, winning the Varden's trust. He then continued to use his hammer as he served in the Varden, using it (and a few spears) to kill 193 soldiers (194 if you count the one he stabbed before being shot by an archer) in one battle.
Eragon later reinforced the hammer with his magic after the battle, making it even more effective. Roran had always preferred a bashing and crashing technique while fighting rather than a smooth slashing and deflecting style.