So far leveling is fairly even. Your part of four level up while they fight, those that are not being used gain some XP while back in town waiting, and if you build a Fort and station guild members in the fort they gain XP even faster yet. I filled my guild as soon as I could and picked a cool 4-man party of Soverign, Ninja, Wanderer, and Medic and put them in a Fort and decided to build a second party and try them out and they were about the same level as my first party! I was filling up my Inn stash with older, hand-me-down gear so I decided to give it all to them so I have a decently equipped and skill point spent squad as a backup.
This is important for rescue missions. So far if a character drops to 0 health he passes out and I usually use a thread to retreat or a means to revive him to keep going (lest the game get harder with only 3 party members and more importantly, to avoid said knocked out character missing out on XP). However, the game explains party wipes, where you then have to form a second party to attempt a rescue mission. This hasn't happened to me yet, so I'm not sure what will happen exactly.
I know Forts play a role in it, but I've build a few forts and still don't completely understand their role, except to keep the rooms in the area from becoming randomized? They still seem randomized to me though when I re-explore around the Forts, because the map is once more blank and fills in while I walk around.
There is a lot I don't quite get, but the game itself and the combat is really engaging! I just opened up the fourth dungeon but I'm using my secondary party to sort of back tread dungeons 2 and 3 and doing some quests I forgot about. Sovereign's are freaking amazing, is my take away so far.