After the fight with Mardracht, I no longer have the option to Save or Load a game. Those options are grayed out. I have tried changing areas, long rested, etc., and the options to save or load a game stay grayed out.
If I quit the game and go back to the main menu, the options to "Continue" and "Load" a game are both grayed out, and hovering my mouse over them tells me that there are no valid save games.
But when I exit the game and restart, I have my full list of saved games, and can load any of them and play. But as soon as I fight Mardracht (I have done this fight 8 or 9 times now), the bug to not be able to save is back.
At this point I am stuck and can not continue the game
The game ends at this point. You should be able to open a save game that was made before you entered the last level of the tower.
There is an infrequent but known bug when trying to save / load within the castle but, in your case, there is nothing left to continue anyway after the fight with Mandracht. As soon as you leave the area surrounding the castle, before you can enter the world map, the game displays a (paraphrased) "Thanks for playing Early Access!" message and exits to the main menu.
Thanks guys, I guess I won't worry about it then. I'll just wait for the next patch and start a new run through!