How to use Testudo formation?

[Incendio]

I am now playing historical battles, I am trying the Siege of Gergovia in hard level and I realize I have Testudo formation available but I don't know how to use it and when is the best time to use it. Can you please help me?

PS. I really like Rome Total War historical battles, they are very immersive.

[Solon de Atenas]

This formation is very good to avoid damage from projectiles (arrows) but really bad for melee combat. So you can use it when you are advancing to walls or receiving attack from archers, etc.

[Incendio]

Thank you. Do you have any tips to defeat gauls at Siege of Gergovia? I only see 2 projectiles units and I think is not worth testudo formation since gauls launch everybody in a melee assault. I tried with testudo formation but I was crushed. I would like to use the 3 cavalry units effectively because I feel most of the battle depends on cavalry (as most of the historical battles in this game).

[Solon de Atenas]

I have no speciphic tips about that. Try to hit with your best infantry the weakest part of the wall/door/unit and concentrate the attack in this area. In case that there are units outside of the wall try to kill them first. Inside of the settlement try to move and conquer by sections and try to isolate units to destry them...