how has this been implented in skyrim?

Sorry to say but the answers will be spoilers, what I will say though is the Soul Cairn is a necromancers wet dream!

I cannot understand the message you are trying to convey

I want to know if the Soul Cairn is a flavourful location, what feel does it give, if you like it and if there is a castle to explore.

I think its awesome, lots of ruins to explore and you can pick up some nice spells in the process. I think it looks amazing and the atmosphere is pretty wild. You can get conjuration spells to summon arvak and bonemen.

thanks. a ruined castle somewhere?

There are castle ruins all over the place and some are pretty good sized.

thanks. once this is out for the pc then I will use the tieset for my Dark Fortress location
is there any floating place there in the soul cairn?

Is it possible to return after the DLC is completed like The Shivering Isles?

Yes

Deep, silent, complete - black velvet sea.
... Alright, I know I used song lyrics.

It is mysterious and eerie, yet also soothing and relaxing in a way. My favorite part of the expansion, and the very reason I'm iffy about removing Dawnguard from my hard drive. (Random attacks are too much.)

The soul cairn is what sovngarde should have been if you asked me(maybe a bit darker). about 5 or 6 quests to do in the place alone, a dead shopkeeper who sells you decent tomes and equipment for soul husks(plat things that grow around the place), soul crevises and portals, puzzles requiring the use of magic, and even brand new enemies that are each unique in their own way.
Plus, the spell summon Arvak is definitely useful if you can get it from here. Who wouldn't want an undead horse to call forth whenever you want?(especially with the addition of horse combat ).

And whats even better, you can return here after the questline is complete! This fact alone makes it twice a good as sovngarde.