Castellan comes in on a three-month loan deal while Robert Eagle re-joins the club permanently after a brief loan spell from Leiston last year.
On signing the pair, manager Jamie Godbold said, "I'm delighted to bring both Matt and Eags to the club. Matt is a ball-playing centre half and will fit in well with the team and the dressing room. He is really keen to develop himself at step three and we thank Kings Lynn for supporting this move."
"Eags needs no introduction to the club. His experience at the level will benefit our squad of players and his quality going forwards gives us a great balance at the top of the pitch."
Both players will join up with the squad for Saturday's away trip to Rushall.
Updated 18:42 - 11 Oct 2018 by Matthew Reddy