Parson has taken advantage of the following things:

a) Realtime command control and intelligence on the battlefield for his side, versus limited command, control, and intelligence on the other side.
b) Using warlords to control combats (selective targeting of enemy units, ability to break off combat).
c) Using restricted terrain and special unit capabilities to prevent an effective counterattack on his weakened A-dwagons.

If I had to guess (and I do), and if Parson has managed to wipe out the siege units already, his next stunt will be to wipe out the weaker warlords in the column using the A-dwagons and wipe out enemy scouts using the B-dwagons, withdrawing the dwagons to Gobwin Knob at the end of the turn. This would wipe out Anson's already limited recon and reduce Anson's ability to control the column. That would improve Parson's ability to stage ambushes and to force Anson to fight battles at unfavorable odds in the future. In essense, he takes two of the advantages that he already has exploited and increases his edge in those areas.

There is also the potential foreshadowing from earlier that Parson is considering how to force Anson to make the main effort underground. If he wipes out the siege engines and if he can obtain air superiority (perhaps through ambushes of air units), then Anson will be forced to make the underground diversion into the main effort. My impression is that Gobwin's Knob's underground defenses may actually be stronger than the aboveground ones ... particularly because it will be so hard to have accurate intelligence of them and because Jillian can be used to pass false information about them (she can "remember" incorrect details about her escape, for instance).