Before I begin, the description on Odallus' steam page lists Ghosts 'n Goblins, Demon's Crest, and Castlevania as direct inspirations, but Odallus actually plays like none of those games. Sure it shares the same kind of oppressive atmosphere (in spite of its versatile color palette), but all three of those games play considerably slower than Odallus does—hell, JoyMasher's game doesn't even have a quirky jump mechanic like those do! Odallus is less about planning your next move and more about executing swift barbarian justice on your foes... kinda like Oniken! In fact, that's probably the best analog to the game: picture Oniken, enlarge the sprites, add in some backtracking and cool powerups, and there ya go!
The backtracking aspect plays a bigger role than the above sentence might imply, but it's important to note that the majority of Odallus is played as consecutive stages from an NES game (with a few branching paths to spice things up). Rarely will you have more than one viable stage to select from, keeping things feeling less Metroidvania-y and more classic, despite the fact that you'll still have to head into earlier levels to pick up goodies or find a secret exit. Thankfully the levels are shorter than you remember upon replay and offer plenty of permanent shortcuts to unlock (like activating a skull-face teleporter), never burdening the player with prolonged periods of boredom. The overarching design is a successful mix of "old meets new", playing like an retro platformer but having enough modern design sensibilities to never become overbearing or groan-inducing.
That said, Odallus can still be a prickly at times, especially once you reach the final boss (which is an astoundingly awesome fight, by the way). It's no easy task to reach the credits but I'd hesitate to call Odallus a difficult game, or at least difficult in the same way its alleged inspirations are. The only thing that conflates its perceived challenge is that the game sticks to a limited lives system despite not really needing to, given how prevalent the shortcuts are. I was also confused for a long time whether or not the unlocking a shortcut would persist after using a continue, but fortunately they do, which I wish the game had indicated to me beforehand somehow. For the most part Odallus is very gentle and rewarding at just the right times (like stumbling upon a full health chest after a section filled with fierce enemies), which is a testament to how tightly designed the game is; as long as you proceed cautiously and keep your side-arms ready, there's no reason you can't reach the final stage in a few hours (good luck beating that level though!)
Besides the limited lives, I suppose I don't have too many qualms with Odallus overall. The controls felt a little sloppy and slidey at times—Oniken suffered the same problem if I recall correctly—but they're easy to get used to by the third level, especially once you eventually obtain the double jump and command more control over your trajectory. I'm a little sad that the sub-weapons don't have an upgrade to enhance their use in the endgame, but whapping guys with just your sword remains fun for the entire duration of the game. The story can be somewhat confusing but the Berserk-inspired theme of the game in conjunction with the revolting Gigeresque pixel art is a super awesome combination that never feels trite or dull. Almost every time I think of something that might make me feel a little lukewarm on the game, there's always some other aspect that I really dig and enjoy.
Odallus is not a gigantic leap beyond their blade-swinging debut, but that's because since their inception JoyMasher has understood what made 8-bit platforming so much fun. Whereas Oniken made me a believer in their work, it was Odallus that cemented them as masters of their craft, doing far more than simply invoking nostalgia with their punchy visuals. It's somewhat hard to recommend the game to anyone that doesn't enjoy the era of gaming that inspired it, but for those of us who yearn for an oldschool challenge aided by some clever design, nifty bosses, and catchy pulse wave tunes, Odallus is right down our alley.