The Mark of Fire prevents damage to your party members from lava squares. While it's not strictly required, it's a big help in Castle Fire and elsewhere. It's certainly easier to get than gaining all the extra Levels you'd need to compensate for not having it.
The Mark is found on the bottom level of three different dungeons. You might expect the Dungeon of Fire to be the place to go to find this mark, but while it can be found there, it's the most difficult path by far.
In the Perinian depths, you'll only need to find a ladder on level 02 that will take you all the way to level 08. Not only is the Mark easy to find, but you'll find Fountains of both Curing and Healing in its vicinity, along with the Mark of Kings. Assuming you can stand against enemies like Dragons and Balrons at all, this is a great place to Level since you can heal after each fight.
Alternately, you can immediately reach level 08 of the Mines of Morinia. The mark is a long trek away, and there are no convenient fountains, so while this is the shortest path, it's not the easiest. If you're confident in your martial prowess, though, you can find the Mark of Kings and a lot of treasure here as well.
Personally, I'd recommend the Perinian depths for the easiest time early in the game. The treasure in the Mines is tempting, but there are other places to find just as much without dealing with Dragons and Balrons.
The Mark is found on the bottom level of three different dungeons. You might expect the Dungeon of Fire to be the place to go to find this mark, but while it can be found there, it's the most difficult path by far.
In the Perinian depths, you'll only need to find a ladder on level 02 that will take you all the way to level 08. Not only is the Mark easy to find, but you'll find Fountains of both Curing and Healing in its vicinity, along with the Mark of Kings. Assuming you can stand against enemies like Dragons and Balrons at all, this is a great place to Level since you can heal after each fight.
Alternately, you can immediately reach level 08 of the Mines of Morinia. The mark is a long trek away, and there are no convenient fountains, so while this is the shortest path, it's not the easiest. If you're confident in your martial prowess, though, you can find the Mark of Kings and a lot of treasure here as well.
Personally, I'd recommend the Perinian depths for the easiest time early in the game. The treasure in the Mines is tempting, but there are other places to find just as much without dealing with Dragons and Balrons.