A free mod for Mount & Blade II: Bannerlord, by iyansky13.

The Overhead Shieldwall – Testudo Formation is a mod that allows you to create a formation similar to the Testudo. It is useful for creating battle formations and allows you to use shields as an asset for your troops.

Note: This mod is in a work in progress, so it might not be working properly.

It is a shield-based formation that uses the same concept as the Testudo, in which the soldiers of a given nation (or army) will form a tight and compact circle, with their shields in front. This circle will be extended or shortened depending on the size of your army, and it can be used in different combinations.