Not sure yet, to be honest.

I am trying to come up with a mechanic that functions as special ability but is not "always on".

Like for the legions - Testudo as a special, so they can declare the special is active and gain the ability and negatives until it is deactivated.

Not sure it will work simple enough to be worth the effort.

An alternative is to add a new order type... but that risks breaking the mechanic we have today.

I will ponder it on my upcoming trip.

You have any thoughts?

I was thinking of something like the following:

Testudo
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Some units have carefully drilled and practiced an ability to arrange themselves and their shields in such a way to provide extra defense against ranged attacks. Historically, the Roman Legionaries were noted for this capability.

Activating a Testudo formation for a unit with this special ability is a special reform order.
The unit is arranged in a double column formation, with the two elements side-by-side in the front, and each successive element in a rear hex-side of the element in its front hex-side. The other front hex of the unit should be facing to the side, out from the main body of the Testudo.

It takes a reform order to deactivate Testudo.

Once a unit enters Testudo, it must remain in the formation for the remainder of the Turn that it activated the formation.  Units that begin a Turn in Testudo, may deactivate the formation at any time during that Turn.

Units declare they are in Testudo formation for that turn are considered to have +2 ranged attack defense, but cannot move more than 1 hex per turn. The only valid order to a unit while active Testudo formation is to Move - they can neither declare a melee or ranged attacks, nor may they Reform - accept to deactivate the formation.
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Thoughts?

That would work for me, as an abstraction.

As another option, could it be a hexstand-by-hexstand option?  A formation of three stands could opt for none, some, or all of its stands to "turtle up" each time it takes a reform action.

--Rich Spainhour