I don’t know anything the historical accuracy, so I’ll just critique grammar stuff.
You connect separate sentences with commas quite frequently when it would be clearer/more accurate to separate them with periods instead. For example, the first sentence: “The robe fell heavily upon his shoulders, over the weeks he had learned to become used to the itch of the material but his sweat seemed to stem within the folds making the garment arid and cumbersome” or “Sitting at his desk he pulled out an envelope from a drawer, the seal had been broken but clearly embellished upon what was left was a mark of royalty”. In both of these examples you are putting two separate sentences into one by using commas. It reads strangely. Making sure you properly separate your sentences with periods rather than trying to force them together with commas would improve flow.
An example where you used the comma correctly is here: “His room was doused in candlelight, causing a medley of shadows to dance upon the subtle masonry”, as the post-comma part of the sentence is not a complete sentence in its own right, unlike in the previous sentences I quoted.
The orders are also written a bit awkwardly. Further punctuation should help to clarify, and separating the orders from Piers’ actions by use of paragraphs may help as well.
Back to the periods instead of commas thing, “What sordid little bastard had managed to defile his orders in such a manner, he stood up suddenly sensing a presence within the room, eyes wide with alert he began to turn when a voice pierced the darkness” was confusing for me and I wasn’t sure what was going on at first. A rough fix could be something like “What sordid little bastard had managed to defile his orders in such a manner…Suddenly, he sensed a presence within the room. He stood up, eyes wide and alert, when a voice pierced the darkness”.
The last paragraph could use a little cleaning up as well. You describe the three men independent of their names, then use their names to describe their actions. That gives me the physical appearance of three men but not what names belong to which person. Then, “The second of the larger men lifted piers to his feet and dragged him towards the window, and out over the frame. Piers glanced back into the room seeing the smaller man take one last look around before jumping out after them”. There’s a simple capitalization typo there, but more importantly the description initially doesn’t give me the impression of Piers being tossed out the window, although that’s what seems to be implied by the third man “jumping out after them”. Some clarity would help here.
Overall, not bad. I’d be interested in reading more! Hope the critique is helpful, and good luck to you.
Have fun writing!