The idea cold be simple, made a building that could represent Helm's deep or at least part of it, just like Dol Guldur, for Rohan and be used in some sense has Dol Amroth's castle outpost, with the next units:
Rohan's soldiers: The faithful and loyal soldiers of Rohan, that fights at foot instead of use a horse. They're stronger that the peasants.
Rohan's spearmen: The faithful and loyal spearmen of Rohan, that fights at foot instead of use a horse. They're stronger that the farmhands.
Galadhrim: The elves archers that Lorien sent to Helm's Deep for help Rohan in his darkest hour. They're stronger than Yeomen Archers, able to change between range and melee, with more power at distance, and upgradable with Rohan's upgrades.
Hornburg's Rohirrim: Elite Rohirrim riders able to use swords and bows. Only can be recruited 3 groups.
About the hero, you can put one of Rohan's heroes here.
As Rohan don't have fortress outpost, I think on this. Also the recluited units can give support to the peasant and farmhands with their veterancy