Dead Cells

Was rather hoping someone would have put up directions, on how to get there...

It's been a while, but if I remember correctly, you'll need to go into an area almost immediately after all the doors at the start of Prisoner's Quarters, this little side area has a key that you'll need to access the Cavern.
From there, my preferred path was Prisoner's Quarters -> Promenade of the Condemned-> Ramparts -> Insufferable Crypt -> Graveyard -> Cavern -> Guardian's Haven.
I know the route is right, I pulled up the maps to make sure, I think the key I mentioned near the start of Prisoner's Quarters is the only extra thing you need to get through the path. You'll need to get the key on the run you plan to make it to the Caverns, after that, it should remain open.

Thanks, found it, Died! Wrong weapons, try something different next attempt...

I take it there is no way to avoid getting this while also getting the "complete with 4 cells" achievement? I'd rather just do the base game, but looks like I'll need to do this as well.

You can avoid this achievement. Just play with four cells active and don't go through Guardian's Haven, instead make your way to the Throne Room and defeat The Hand of the King as normal. I'm not sure if getting this achievement would make the "complete with 4 cells" achievement pop or not.

Yep you are spot on thankfully. Playing on 4 cells it says will unlock the fifth, but you don't obtain that from beating the Hand (I guess you have to take the alternate path, as you say).