I think even if I will place a giant red arrow some players won’t be able to understand where to go next. Yorna doesn’t have any planned updates. I may release a cumulative improvement pack later but it won’t happen in the foreseeable future since Ouro is getting all of my time.
I figured there wouldn't be updates for a while.
Though, if you name the Ancient Tower map Mortimer's Tower, or have mentions of Mortimer's Tower replaced with the Ancient Tower, that would help.
Specifying that it's past the snowy mountains would help, too, as when I look at the current overworld, I see the north mountains, southwest mountains, and southeast mountains. The two southren mountain ranges have visible towers.
Being able to talk to NPCs that give additional information could also help… To be more to the point, due to my experience with fortune tellers in RPGs, as well as Legend of Zelda, I find myself going to the fortune teller (in the town with the seamstress) for guidance when the help command isn't very descriptive\useful and I can't find the relevant location\NPC for the quest… Only to find that their dialogue hasn't changed.
In either case, these are just free suggestions. I eventually figured it out through a combination of research with trial and error.