This is the only episode in the entire series that ends on a "cliffhanger." (A "to-be-continued" scenario where the following episode picks up precisely where this episode ended.)
When Metellus questions Crassus as to why he hadn't attacked Sinuessa, Crassus retorts that Spartacus is a man of keen intellect and would not be overcome by brute force. Unlike all of Spartacus' previous Roman foes (who only saw him as a brutish slave,) Crassus saw him as an equal; someone to be maneuvered around through cunning. This is what made him Spartacus' most deadly adversary because he did not underestimate him and actively sought to outthink him.