By my count, Dragon Quest 11: Echoes of an Elusive age is the second, big-name, secret door-filled JRPG to come to PC this year. There are two varieties of secret doors in Dragon Quest 11: silver doors which can't be opened until you finish the main story, and red doors which can't be opened until you reach a certain point in the main story. We're going to focus on the red doors for now because there's a lot of good stuff behind them and you can open most of them well before you finish the game.
Most red doors are hiding treasure chests containing armor, weapons or crafting materials, but some will also get you priceless crafting recipes. There are 15 red doors in all, and to open any of them you're going to need a magic key. I don't want to spoil too much, so I'll just say that you get this key after a certain boss fight in an area called Phnom Nonh. You'll automatically receive the magic key after defeating the boss. This is all part of the main storyline, so you can't miss it.
Once you have the key, it's time to track down all 15 doors. There are 12 scattered around the major mainland areas, and three more are tucked away on small islands. Luckily, red doors appear on the map as a red lock symbol even if you haven't opened them. However, they only appear on your local map. They won't show up if you navigate to a far-off area via the "nearby areas" map menu. Here's where to find them all:
- Cobblestone (aka The Last Bastion): along the river on the east side
- Downtown Heliodor: a house on the east side
- The Hotto Steppe: in front of the crypt on the west side
- Gallopolis: a house on the east side of the racetrack (this door should be a priority)
- Gondolia: a house on the west side of the main bridge (this door should be a priority)
- Zwaardsrust Region: down the stairs west of the inn (this door should be a priority)
- Dundrasil Region: a small hut on the east wall across from the ruins
- Puerto Valor: a house on the east side near the church
- Lonalulu: along the western wall of the eastern path
- L'Academie De Notre Maitre des Medailles: downstairs to the northwest
- Phnom Nonh: on the second floor south of the armor shop (this door should be a priority)
- Sniflheim: outside the city to the northwest
- Insula Australis: on the island northeast of Gondolia's port (accessed by boat)
- Insula Centralis: on the eastmost island (accessed by teleport)
- Insula Occidentalis: on the small island north of The Strand (accessed by boat)
Thanks to PSU for helping me fill in the blanks for this guide.