Magic Key Door Locations - Dragon Quest 5
The Magic Key is the first one that you'll acquire in Dragon Quest V and it's found in the town of Lofty Peak which you can only access during the third and generation of your Hero's life/the game. Use the L + R buttons to change the camera angles while you're in this town and search around back of the large central pillar that goes through the center of town. Here is where you'll find the treasure chest that contains the Magic Key.
In total there are 4 optional doors that you can open with the Magic Key, the final door in the town of Fortuna isn't even really worth your time since your reward for opening it is only knowledge. I tell you what the guy says anyway at the bottom of the page to save you time from going there.
Magic Key Door #1 Roundbeck
Items: 15 Gold Coins, Stone Fang, Mini Medal (in pot)
Go into the pub in the northwest corner of town and tell the barkeep at the counter 'No' to a drink. She'll open the door for you and tell you to speak with her father, instead what you'll want to do is go into the basement and steal all their stuff.
Magic Key Door #2 Lodestar Harbour
Items: Thorn Whip & Seed of Life
You can find this door by entering the inn and going behind the dancer's stage during the day. Go down the hall to the right to find the locked door.
Magic Key Door #3 Gotha Castle
Items: 3000G, Meteorite Bracer
This Magic Key Door is found on the second floor of Gotha Castle right next to the pub where Patty is. Probably one of the easier Magic Key Doors to locate and one of the first ones you'll probably remember from earlier in the game.
Magic Key Door #4 Fortuna
Items: N/A
At the northern end of Fortuna you'll find a staircase leading down into some jails. It's just to the right of the house where you purchased the Wagon earlier in the game. There will be a man in a jail cell here for you to speak to and that's it.
Fortuna's Magic Key Door is not even worth getting. Your reward is a man that tells you in the Casino on the blue side the second from the left in the back row has a better payout. According to Google searches the pay out increase is only very minor. There is no other reward or flag for discovering him, just this piece of information.