[ Arthur's armor generally gives himself away as a Knight of the Round Table, at the very least, but he's unaware that Archer knows one of his female counterparts rather well. If he'd known that, he would have been less surprised when the other Servant figured out who he was so easily. If Ritsuka was close to guessing just from her brief experience with the female Alter, after all, surely someone like Archer with such a sharp mind would have been able to get it in one.

Then again, with Ozymandias around, it is difficult to keep his identity entirely a secret anyway, so who knows how the old man figured out who he is. Only he would know. ]

My chivalrous heart is not necessarily stronger than my suspicious mind.

[ He frowns at the other Servant, but at least he does stop squirming. ]

And you are a Servant, if an older one in appearance. Were you not summoned in your prime? As one of the three knight classes, you are hardly defenseless yourself.