Spartacus creative solution

Spartacus had been surrounded and locked by the Roman cohorts at the top of Mount Vesuvius. A huge army stood in front of  him and a deep chasm was behind. The current situation suggests “only one” obvious way out – a decisive battle. His soldiers prepared to it through weaving shields from withes. Strolling around the camp, a brilliant idea flashed across Spartacus’ mind.

What did he decide?