DKS is the remastered D2:ED plus FoV. You can start a new level 1 character and go straight through to the end of FoV, or start a new level 35 character at the beginning of the FoV section of the game (primarily of interest to those who already played the original release of D2:ED).
The Hall of Echoes is the final section of the D2:ED part of the game; getting out of there is the start of FoV.
While you can return to the battle tower in FoV, the teleporter shrine will be disabled, so you can not get back to Broken Valley, the fjords or the exterior of Sentinel Island.
