I just wondered. Looks like it's possible, but at least BT2 & BT3 need certain characters. The required Rogue appears to be not exactly favoured by most people. That might be a problem among many.

So far I just have some first impressions of Skara Brae, both with a party and a solo char. The tactical depth should profit from a party, having a composition and back row characters for instance. The Adventurers Guild's options however really make for interesting formats for a solo I reckon! I toyed around with the idea of changing up characters, so I'd bring one at a time only and yet I could use different classes. It might be fun to explore the possibilities of summons and wandering monsters, too! Then again the Bard is kind of the heart of the game and mostly bolsters a party. Perhaps useful with the right wandering monsters, but not exactly the ideal solo char.

So anyway: Is solo'ing a thing?

I have worked out what the minimum requirements are. (TL;DR: BT1 and BT2 can be theoretically soloed; BT3 can not.)

BT1: There is one puzzle that can only be solved with a Bard in your party. However, it turns out that PHDO and APAR work on this floor, allowing you to simply bypass the dungeon if you're playing a spellcaster. There's a certain boss that requires a special weapon to kill; in classic versions, carrying the weapon is sufficient, but in the remaster, you actually have to inflict the killing blow with the weapon, making it impossible to get past without a character who can use it (spellcasters can't, but Bards can). Furthermore, you can't go back to the Adventurer's Guild after the puzzle but before the boss. Therefore, this game can be soloed with either a Bard or a spellcaster who gets either level 7 Conjurer or level 6 Magician spells, and the remaster can only be soloed with a Bard.

BT2: The Dark Domain requires a Bard to complete, but completing it is optional. Only an Archmage can become the Destiny Knight, so having a spellcaster who becomes an Archmage at some point is required to beat the game. Hence, the game can be soloed, but only with a spellcaster.

BT3: This game, however, has stricter requirements. Specifically, you need:
* A Chronomancer to go to any plane other than Skara Brae; you can't progress at all without one.
* A spellcaster who does not become a Chronomancer; specific spells are needed for a puzzle in in Gelidia, and Chronomancers don't get them. (Changing a spellcaster into a Chronomancer causes them to lose all their former spells, and you can't reach Gelidia without a Chronomancer.)
* I think a Bard may be required to enter a dungeon in Lucencia, but I'm not sure (and there may be a version difference here).
* In the final dungeon, you need to use certain items that are class restricted (but I'm not sure if that check is enforced in all versions). You need a Rogue for one item, and there's at least one that requires a Warrior, Paladin, or Geomancer. (Note that it's possible to permanently lose the ability to make a Geomancer if you kill a certain someone who doesn't want to fight, and that Geomancers can only have Geomancer spells (and aren't available until after Gelidia, anyway).)

So, BT1 and BT2 are theoretically soloable, but BT3 is definitely not (you won't get past Gelidia if you try).

I see, so it's a bit more complicated...
Sounds like the game didn't really consider solo'ing a thing. Perhaps I should save that for a 2nd playthrough, so I know what I am doing and where I am headed at least. From where I stand Bards don't look like the ideal class to brace all the challenges the game throws at me by himself, so that may be a tough challenge.

Thank You for the detailled information dtgreene! It is highly appreciated <3