I think it's referring to someone (possibly Kaladin) climbing the shield (the wall of grief) around Urithiru (or the outside of the tower) to gain access in some form. "light, locked away above" could refer to the Sibling (and Towerlight) being placed in the inner shield with Navani (the captive).
If it is Kaladin, the darkened twin sounds like Moash.
I don't think Kaladin would love Towerlight though, unless the inhabitants of the tower have figured out how to split Towerlight to produce Stormlight?