In this theory, Sen's Fortress was supposed to function as the royal armory at some point due to the stacks of unused Silver Knight statues found. It is known that Gwyn's Firstborn was a god of war and, aditionally, we learn in Dark Souls III that Gwyn's Firstborn trained Ornstein to be his first knight. Ornstein eventually became the leader of the Four Knights of Gwyn and presumably the rest of Gwyn's Silver Knights. This implies that Gwyn's Firstborn was originally the leader of Gwyn's knights before Ornstein took over, and presumably would have control of the royal armory in that position.
Additionally, Serpent Men are found throughout Sen's Fortress, much like in Archdragon Peak, the location where the Nameless King went after aligning with the dragons. Interestingly, the Covetous Gold Serpent ring is found in Sen's Fortress, hidden behind a destructible wall. Serpents are said to be imperfect dragons. Perhaps this was a hint, along with the Snake Men, of Gwyn's Firstborn's secret sympathy for dragons.
We know from Andre of Astora's dialogue that the fortress was built by the ancient gods:
"Sen's Fortress is an old proving grounds built by the ancients gods.
It is the only route leading to the great Anor Londo."
It is possible to deduce from this dialogue that Sen was an individual from the age of the gods - possibly either the builder or owner of the fortress. Otherwise, the name Sen is never mentioned specifically outside of references to Sen's Fortress itself. This is rather conspicuous, as many gods and members of royalty are referenced through item descriptions who have far less influence on the world and plot of Dark Souls - Flame God Flann, Old Man McLoyf, Allfather Lloyd, etc. Despite the lack of references to Sen as an individual, the name of Sen's Fortress persists.
Andre of Astora, is the only character in the game who definitely knows its name. This implies that the fortress:
1) Has existed for a very long time, like he stated in his dialogue; and,
2) Is significant enough for its name to be remembered by Andre, though the name is largely forgotten by everyone else.
Frampt and Crestfallen Merchant also know the name of the Fortress
Andre of Astora appears thousands of years later in the world of Dark Souls III. This implies that he may have been alive when Sen's Fortress was built, before the exile of Gwyn's Firstborn. His age would explain how he remembers the one name (Sen) that everyone else has forgotten.
With all of this (circumstantial) evidence at hand, it seems likely that Gwyn's Firstborn/The Nameless King is Sen.
https://www.reddit.com/r/darksouls3/comments/4k4n5f/lore_the_nameless_king/
the nameless king is just ornstein, i started thinking so because of several facts:
1 you find the dragonslayer spear in the gate connected to the after-bossfight zone
2 in the bossfight zone you find the dragonslayer set
3 nk has some ornstein's movesets, like the bash
4 you fight the dragonslayer's armor insteed of te dragonslayer at lothric's castle, that is just an empty armor
in fact ornstein, after being killed during ds1 has come back in ds2(probably as an undead), but he is not protecting anymore the princess, i think that after being killed for the first time he leaved his role as a guardian and dragonslayer and then decided to ally whit the dragons. reading the nk's description in fact we can read that he was an old dragonslayer, like ornstein. and in ds2 he is called "old" dragonslayer, i think that this is not just an age referral, because he doesen't seem really old too, but it's referred to the fact that he "was" a dragonslayer and now he's just a simple fighter, until in ds3 he went to the archdragon peak and decided to ally whit the dragons, rejecting everything he was
the thing is that I think it is more logic that the nameless king is Faraam AND Sen, in dark souls 2 we know that some gods are known by differents name dependig the region, plus he got rejected by gwen but still is the legitimate king since you must kill oceiros to access to him and Ornstein is no king and finally, (I don't remember if what I will say if facts or theory) Ornstein, like Havel, were with the nameless king before his treason, then Gwen made everyone forget him by destroying statues of him (because he's also probably the god of sun) and after that, he placed Ornstein with Smough not to protect Gwen's daughter, but just an illusion, which was really degrading for him since Smough is a cannibal executionner and that his task was'nt that important