Slitherine

Historically, where did these units get deployed on the battlefield. ?

Is historical placement the best way to use them in the game?

Sometimes they were left to guard the camp, other times (such as the freed slaves fielded in desperation at a low point in the 2nd Punic War, and equipped with previously captured Gallic weapons) they fought in the line of battle.

In the game they are best kept in reserve. They are quite good for fending off enemy cavalry who are trying to get round your flank, but then so are triarii. You certainly don't want them as part of your front line.

I usually have them (and raw infantry in general) in third line of checkerboard style formation or even the second if I really need to stretch the line.