Great Dracula X-Metroid combo
You really can't throw a stick without hitting Metroid-styled indie games these days. This, of course, is a good thing. Odallus falls into this camp, with some differences. It's more akin to Dracula X with the alternate stage paths, and Metroid with the permanent upgrades. The ever-classic Demon's Crest is a good example as well.
While taking the role of Haggis, you'll come across the usual abilities: double-jump, float, dash, and so forth. This makes it necessary to revisit previously completed stages to suss out all their secrets. Some are quite the bit tricky to find. Combat takes a bit to get used to. Your initial sword swing is pretty fast, but there's a significant recovery period after, which leaves you open to attacks. Combat in the early going, in particular, can be fast and brutal. If you can track down the health and armor upgrades, things get easier, but they never get too simple. Haggis controls well, for the most part, although some of the physics don't feel completely right, much like JoyMasher's previous title, Oniken. Still, it's nothing that really gets in the way of enjoying the game.
Levels are pretty varied, although fielding so many underwater areas is a bold move. It lends to the atmosphere of exploration, but your movement rate is pretty slow there. The graphics are quite excellent, with good spritework and a feel that approximates the NES, although the larger sprites and multiple layers of parallax scrolling will have you thinking TurboGrafx-16 instead. The soundtrack is solid, as well.
The plot is pretty dark, but it's really not overly important to this type of game, much like NES games of the time. It's the exploration and upgrading that proves addictive. There are a few slick-looking cutscenes along the way, done in the style of Ninja Gaiden, and they are much appreciated.
Really, if you're looking for a solid, NES-style action-platformer, it's hard to go wrong with Odallus.