"The Aquilonian host was drawn up, long serried lines of pikemen and horsemen in gleaming steel, when a giant figure in black armor emerged from the royal pavilion, and as he swung up into the saddle of the black stallion held by four squires, a roar that shook the mountains went up from the host. They shook their blades and thundered forth their acclaim of their warrior king - knights in gold-chased armor, pikemen in mail coats and bassinets, archers in their leather jerkins, with their longbows in their left hand."
"The Aquilonian host moved leisurely to meet them. The measure tramp of the armored horses made the ground tremble. Banners flung out long silken folds in the morning wind; lances swayed like a bristling forest, dipped and sank, their pennons fluttering about them."
Here is a quote about Aquilonian battle formation...
"The Aquilonian host had assumed the customary formation: the strongest part was the center, composed entirely of heavily armed knights; the wings were made up of smaller bodies of horsemen, mounted men-at-arms, mostly, supported by pikemen and archers. The latter were Bossonians from the western marches, strongly built men of medium stature, in leathern jackets and iron head pieces."
I think these three quotes are the most important in showing that Aquilonia is actually a High Medieval culture similar to France or England possibly late 14th or 15th century. There's nothing here to indicate Aquilonia is based on Rome. No mention of legions, shield walls, segmentata armor, pilum etc. Instead we have an army that relies on heavily armored cavalry Rome was for the most part reliant on infantry with cavalry in supporting roles.
The best resource is always the original stories. I realize the name Aquilonia has a Roman ring to it but if you throw out pastiches, comics and artist interpretation of what they want to convey about Robert E. Howard's world, there's no way you could get Rome out of Aquilonia.