The preceding scene ends with a close up of a sewer underneath the great wall of Helm's Deep.
We hear Saruman's voice and then see Saruman speaking to Wormtongue about this sewer, which he says is Helm's Deep's one weakness.
We see him pouring out some black powder, making sure Wormtongue gives it a wide berth with the candle he's holding.
Wormtongue insists that it would still take an army of thousands or tens of thousands to breach the impenetrable fortress.
Saruman agrees, and then walks out to the balcony of his Orthanc and addresses the enormous army of orcs and uruk-hai he's been gathering. Wormtongue is stunned.
He tells them to march to Helm's Deep and leave none alive. We have a feeling this is going to be a long night.