This Seeking Tech guide will show you how to find the Rebel Camp, which is the only missable location in Final Fantasy II Pixel Remaster.
To begin, you need to play through the story up until you rescue Princess Hilda in the Coliseum. Once you do, exit the area and then return to the world map.
Afterwards, you should immediately travel slightly west of Gatrea.
Optionally, you can return to Altair, where you can then hear one of the NPCs talking about where Prince Gordon and Princess Hilda went.
You need to visit this place before you head to the next point of the story, which is to retake Castle Fynn. So, the window to visit the camp is extremely limited if you aren’t careful.