The Mark of Force allows characters to pass unharmed through force fields. If anyone in the party does not have this mark, no one can pass through the forcefield (though only the first character without the mark takes damage from the attempt). You'll need this mark to find Exodus in Castle Fire, and it will prove useful elsewhere as well.
There are two dungeons where you can find the Mark of Force, and neither offers it up easily. In the Dungeon of Fire, you'll need to trek through five long dungeon levels and some traps to find the mark. The trek isn't as long in Doom, but you won't skip any levels so you'll spend more time dealing with nasty enemies. Not only that, but Doom offers no other Marks (though the other mark in the Dungeon of Fire can be more easily obtained in other dungeons). Both choices are valid.
There are two dungeons where you can find the Mark of Force, and neither offers it up easily. In the Dungeon of Fire, you'll need to trek through five long dungeon levels and some traps to find the mark. The trek isn't as long in Doom, but you won't skip any levels so you'll spend more time dealing with nasty enemies. Not only that, but Doom offers no other Marks (though the other mark in the Dungeon of Fire can be more easily obtained in other dungeons). Both choices are valid.