Summarizing the Third Century Crisis

[jho]

In the Gallic Empire, defense in depth was developped; Gallienus and the Palmyrene leaders preferred to put more soldiers on horseback.

I believe that the concentration of troops at central locations was more due to the fact that the emperors needed them to fight their fellow Romans and also needed to stay in personal contact with them to survive than to any changes in defensive strategy. With respect to the "increase" in the number of cavalry, evidence is shaky and it has been pointed out that the strategic speed of cavalry armies is far lower than that of infantry armies. Again, this may be more of an issue of concentrating "high value" troops closely around the emperor.

Re: the defense in depth aspect, the Augsburg stone that tells of a Germanic raid that was intercepted on its way out of the empire is a beautiful piece of evidence.

Given the ragtag nature of the force that defeated the Iuthungi (apparently including local militias), I am not sure that it can be taken as evidence of any sort of policy.