First and foremost, don't force full-screen. I've noticed this often with RM games, but fullscreen usually results in bars phasing through the image at times, and your game is no exception to this. If people want to play full screen, they can hit Alt+Enter.
Secondly, the BGM is too loud; it's drowning out the sound effects. There's also some clipping where the song hits a its high peak. Either adjust the volume in your audio editing program, or—if that clipping is intentional—lower the volume when setting the BGM.
No introduction? Just BAM! thrown right into the game? At least some kind of introduction—like about this prophoecy that someone will appear during the hour of need—would be nice.
You can select "Nevermind" from the list of choices right away, rather than having to go through the questions first. Which leads to Tyrin remarking if he got here while sleeping, he must still be sleeping. Howm does he know he arrived while sleeping if he never asked how he got here? Change it up somehow so you have to ask the questions before getting the forth option.
"I have something to show you. Not THAT you perv." Yeah, I wasn't thinking about that, and I highly doubt Tyrin was, either; he seems too concerned trying to figure out where he is.
Not to mention I was able to go downstairs and continue on with the game (didn't even need to talk to Mia) before going upstairs to get my equipment. And even when I did go to the third floor, I went straight to the fouth floor before being told to. And then I restarted the game and discovered I didn't even need to talk to Mia on the second floor where I started. It's funny to have Mia in my party from the fouth floor and then talk to her on the second.
Do something about those stairs teleporting you face-first into walls, please. You've left directional facing on RETAIN rather than facing away from the walls. Not to mention on the lower levels, they're leading into the walls, while the upper levels are facing away from the walls (in other words, the staircase-in and staircase-out are facing opposite directions).
Losing to Mia results in a Game Over? What the hell? Pretty harsh for a tutorial boss, don't you think?
So, I finally get to the castle, and I enter it sideways. And I'm teleported into the entrance, sideways. You really need to work on the transfer points. And on that note, you should also place the transfer-to location one tile in front of another transfer-from location. So that way people can just turn around and go back if they don't want to be here, instead of having to move forward and then turn around.
While I was in the castle, a ghost treasure chest started stalking me.
And then some ghost lamps started following me.
And then all sorts of ghosts started chasing me!
Typing of the lamps, when I hit the action button when facing a one, it sorta dances. I'm thinking you don't have the "Direction Fix" enabled, and so the lamp is trying to turn to face me whenever I hit the action button, but you don't have anything programmed for it to do. So it just dances and stuff.
I noticed the ghost phenomenon is somehow linked to events, and for whatever reason, the events begin chasing me at the edges of the screen (they disappear after a battle, though). And then I also noticed that the ghost events will also sort "jump into place" when their location comes into frame. Really bizarre stuff.
By the way, you're trying to make a vertical pillar using the top halves of pillars. That doesn't work, because they have a passability.
You have two locked doors. One reads "Locked" and the other reads "LOCKED." Please have some consistency.
If I was the Slime Lord, I would be very pissed off if my underlings ran away from me.
I'm interested to know what dirty magazines from our world are doing in this alternative world, and why Mia knows what Playboy is and/or why she would be interested in Playgirl if she's got a problem with perverts. I'm starting to get the idea that "Familiar of Zero" has influenced this game (as if the earlier part about a hero with no prior training of swords would be able to freely wield weapons wasn't already a hint).
I can talk to the Prince before talking to his father. I guess that's not necessarily wrong... though it is strange.
Your rounded doors open up into tunnels. I mean, they have the little side-bricks around the arch of the tunnel. So, your door is opening up into a hole that's smaller than the door size.
Your little girl who wants to go see the treasure mountain seems to be playing a game of red light/green light. And I applaud her to be keeping balance on one foot.
I can walk all over the upper half of Grune Town on the overworld map? Must make the peasants in that part of the town frightened to have their homes stomped on by a giant.
The girl explaining Grune Day and free Grunes loses her face.
And the game froze when I went into the Granny's house with a bunch of flowers. And I decided that was enough of the game for now.