Preston keeps bugging me to take the Castle. I understand this involves fighting a big nasty something or other.

What level should I be before attempting this?

Get Danse to help you out, his power armor can soak a lot of damage. I took back the castle pretty early, like level 16 or something. It's more dependence on the weapon you have on rather than levels.

Make a hard save, try and see. Jet will give you the oomph to kill any single enemy 3 times over. I may have even died on jet the first time but then was more careful.

They have a flamer and a missile launcher at the castle.

I suggest a Fat Man, MIRV, or a Big Boy though.

Preferably a Big Boy MIRV tbqh.

Its only the second time I used PA. I did it much later and did not have any real problems. Yes some huge Mr Krabs (Spongebob ref for those NOT in the know) along with a bunch of his friends have taken over the place so you will have to forcibly evict them.

You could try laying down mines before the Mirelurk Queen turns up. I did it at a relatively low level and died a few times at first, but it was do-able in the end.