Hijiri in the Succubus Castle is a "Search-Action", or "Metroidvania" type of game that features the character Byakuren Hijiri from the Touhou Project franchise. As it is an adult game, it also features a lot of porn.
Story-wise, the game wastes no time setting up Byakuren suddenly waking up trapped in a "Dream castle" where a Succubus Queen seeks to torment her. Although it isn't initially made clear what her intentions are, should the player falter and lose all of Byakuren's health during later boss fights with her, she makes it rather clear her goal is to turn Byakuren into a lust-driven woman who can breed more minions for her.
However, no matter how many times Hijiri falters in-game, she will stay resolute in her goal to defeat the Queen: her "lewdness" only affects the ending, although it does also affect dialogue during sex scenes too.
Gameplay-wise, as a porn game, there really is little reason to ever fall to the enemies: in other words, Hijiri's got no reason to get a sexual awakening in this game, and she never truly consents to having sex with anything. This includes some of the endings where her corruption by the Succubus drives her to uncontrollable lust.
So if you're looking to succeed in this game and get rewarded with Byakuren riding men to show she's a dom perfectly comfortable with her sex life to the succubus, well, sorry, she's strictly a sub here, and only in defeat, no less. You do unlock a "gallery" after beating the game, so you don't have to manually fall to every type of enemy to see the scenes, but there's no gallery for the endings as far as I can tell, unfortunately.
Incidentally, Byakuren can get pregnant in the middle of the game. It's not made clear if she's impregnated when a monster cums inside of her, though it seems to be likelier the lewder she is. If I had to put odds on it, I'd say it's a 1 in 3 chance. Regardless, pregnancies do not affect Byakuren's gameplay and progress quickly as Byakuren travels past rooms, so you can have her in a knocked up belly state fairly quickly if you just move in-between rooms repeatedly. However, Byakuren will hold off on giving birth indefinitely in-game until she loses all health again (which overrides whatever scene you would have had otherwise).
Besides the monk musing that giving birth to demons is what the Succubus wants, there's no major consequence for giving birth in-game multiple times. It does affect the ending if you finish it while pregnant though, and to top it all off, it changes depending on what you're pregnant with.
So that right there is several scenes that you can only ever see by getting knocked up by the right enemy and then beating the final boss when the pregnancy is confirmed. A bit frustrating if you're trying to see everything the game has to offer in terms of porn.
Ah, finally, I'm sure some people who are fans of "corruption" might be disappointed to hear that Byakuren only falters to it in the endings. I was sure that reaching 10000+ lewdness, giving birth 8+ times and having high sensitivity in all body parts would make her "enjoy" the in-game sex more, but no, besides having hearts in her moans, she never truly surrenders to the pleasure.
Speaking of the scenes, they're somewhat slow: you can skip dialogue with Z and the scene entirely with S, but fast-forwarding the action is not an option and it kind of drags out. While it's nice they have some rudimentary animations, it makes the porn a bit weaker than it could have been as a consequence. And there's no voice sound effects either, so that's another missed opportunity...there's also only a couple of scenes that have Byakuren move to a different area as a consequence of them, so the immersion is also usually weak if that matters to you.
Now, I'm making it sound like these scenes are really bad, but they're otherwise decent if you don't mind semi-amateurish art. A bit samey in pacing, I guess.
Ok, enough about the porn part of the game, what about the regular gameplay itself? Well, it's a decent action platformer overall, but the physics are a little on the rough side, likely due to the game being made in Wolf RPG. Expect a lot of nonsense hitbox moments against some enemies.
Now, if you're a fan of Metroidvania games with sequence breaking, then you'll (probably) like this one: after beating the first boss and getting the double jump ability, the game essentially splits wide open like Byakuren's legs when I romance her in eraTouhouK, and you can do pretty crazy things like getting to Minamitsu before Ichirin, or challenging the Succubus Queen before meeting any other boss.
The game acknowledges this too, as there's a scene where the Succubus Queen is meant to throw shade at you on a throne, but with proper maneuvering around the map, you can approach her from behind and catch her off-guard.
Other gameplay quirks are numerous clothing options, like Byakuren's biker outfit, swimsuits, an office lady outfit, or even the outfit of the Succubus herself.
Now, the controls can be a bit annoying, and what bothered me in particular was that there's no map button shortcut. You can warp anywhere you want with the map, the only caveat being that (besides special areas like boss rooms blocking warping completely) you can't warp in the middle of a long hallway, only at the "entrance" or "exit", so I wanted to check the map very often, and having to go into the menu first really ticked me off. Inventory also becomes quite a mess if that is a pet peeve of yours, but you can thankfully switch stuff around with C to rearrange it to look nicer.
Difficulty-wise, I found the game to be quite easy: there's a leveling system designed to make you basically invincible if you power level even just a little bit, and there's a hall with a bunch of bikers that you can farm for easy EXP because when Byakuren herself rides her bike, she can become an invincible collision box as long as she has magic left.
In the end, Hijiri in the Succubus Castle is a fairly acceptable Metroidvania that has sadly very little proper gameplay integration for its relatively average porn (besides for the ending).
Overall Rating: 3.5 (Rounded up to 4)