The Castle of Nightreign confirms my own suspicion

Stormveil current state has nothing to do with Godwyn, the the marrows and thorns are due to the Ulcerated Tree Spirits formed by the graves of Sun Realm soldiers buried underneath. In the DLC the Ulcerated Tree Spirits drop Scadutree Thorn spells and they are also found near Enir-Ilim

Similarly, the Castle underground is the grave of the young maidens killed by the cutting-gifted tribe to mimic the proceas of creation of Runes. The Castle as result of that doesn't grow thorns but silver bark, just like the cord end is a silver imitation of the golden braid or red braid

Nice touch: The Castle is growing silver barks because of what is in its bottom