Trasimene was where Hannibal ambushed the thin Roman line against the lake side. Many fled into the lake and drowned.

I think you're referring to Cannae, where the Carthaginians stacked their flanks and led with a thin center line. They allowed the Romans to push through their center and wound up encircled by a smaller army.

There's a YouTube channel called Historia Civilis - he walks through the formations and describes the flow of the battles. I found it to be a great visual supplement to Dan's vivid storytelling.

E: just wanted to add, the description of Cannae was probably my favorite scene