I suspected it, but the King Arturia I know didn't train her magic to such lengths as to be a Caster. Magic was...well, not strictly forbidden, but King Arturia had a sour taste for it. Especially after everything with Morgan le Fay.

This Arturia seems to be slightly different. Perhaps in her timeline she focused on magic instead of sword skills.

[It's hard to look down at Dazai like this, so Bedivere sighs and lets his head rest on the pillow while he looks up at the ceiling. This really is a mess.]