When a quest sends you to look for Mortimer's Tower in the mountains, it actually means you need to go to the Ancient Tower past the snowy mountains.
This miscommunication had me confused for quite some time…
And even after I read a comment saying it was in the southeast corner of the map, I spent most of my time looking in the sunken version through the Kitsune's portal looking for more passageways that may have not been obvious because of the indication blending in to the scenery.
Only when I accidently pressed A instead of Y by accedent while in the Ancient Tower did I actually get some message saying I was looking for something. And only when I checked the other rubble did I find a way to progress.
Apparently I couldn't get any hints from people in the lab, nor was the "Help" command actually very helpful.
Could you kindly address this in the next update?
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.