Depending on your power armor and the weapons you have then taking the castle is really easy. its just a bunh of softshell mirelurks, mirelurks, mirelurk babies and the mirelurk queen. when it comes to the mirelurk queen she is a really hard enemy to fight if you dont know how to deal with her as she has an acid spit attack that just melts your armor. i recommend you to try and stay at a far distance or to use a ranged weapon that has a high DPS with some good damage. like a minigun.

Retaking the castle would be worth it, its probably the best "headquarters" colony. And it should just be easy to retake it

Preston was an odd nut. The only interactions ever available in my play was his marking my map for 'another troubled settlement', eventually retaking the Castle, and then continuing the settlement warnings. Evicting the mirelurks is not as horrifyingly difficult as it might seem. It's a solid base of operations, and makes him happy. It's also fun to arm the place with artillery, basically in the BoS's backyard.