Playing Dark Souls has helped crystalize in my mind a lot of how I want the character advancement options to work in The Final Castle. Back when I was working on the Hexagram rules, one of my main goals was to support flexible cross-class abilities without complexity or undermining traditional class archetypes, though now I find the particular approach I was working on somewhat unsatisfying. There was too much discretion, not enough structure, and the lists of system options were too long.
The Final Castle has a far simpler, and more elegant, method of advancement which I believe satisfies my original requirements. Characters may advance potentially to level ten, and each level gained allows the increase of one ability score* (though the same score may not be increased over subsequent levels). The ability scores are combined with a class bonus (which is half level, round up) to determine most action resolution. So, for example, a character is going to roll something like 1d20 +dexterity +fighter (shorthand here for fighter class bonus) when making a combat roll. Starting stats range from 0 to 3, a given stat can be increased up to +5, and the class bonus rises to +5 at most, yielding a nice range of bonus for even the luckiest and most focused character (up to +13 on the d20 scale at level 10). Such specialization comes at the cost of flexibility, as will become clear momentarily.
It may seem at first glance like this does not have much to do with the previous discussion of Dark Souls. However, like Dark Souls, the magic rules apply to characters of all classes. That is, a fighter, for example, rolls 1d20 +magic +magician when casting spells, and the number of spells that can be prepared is also governed by those numbers. (Recall that intelligence has been replaced by magic.) Now, in the case of a fighter, +magician (the class bonus) is always going to be zero, but +magic may be increased (if the fighter wants to dabble in magic) during level up rather than one of the physical stats. Magicians have access to more methods for learning spells, but any character with sufficient stats can at least learn spells from a teacher, and any character has the potential of sufficient stats through level up choices.
Cleric magic (called boons), is handled similarly, with +charisma and +cleric taking the place of +magic and +magician. Rather than learning spells one by one as does a magician, clerics are granted access to a full suite of powers upon making a covenant with a given immortal. The default covenant available to clerics at first level is with The King of Life**, but other covenants may be discovered during play and accessed by any character that has sufficient charisma score. Most immortals will not covenant with characters that use magic though, as such is considered presumptuous and hubristic. More than one covenant at a time is impossible, and breaking a covenant may come with serious consequences.
* Oversimplifying slightly for clarity.
** Inspired by Dogs in the Vineyard and used with permission.