[Reviewed v0.8 as of 2/17/2022]
Nayla's Castle is a nice little game. I wish it played up some fetishism other than futas a little bit more, but otherwise, it's alright in my book.
Combat is 2D side-scrolling with a little hopping and dashing added in. There's also a slightly longer-ranged "charm" attack in your repertoire, which on many enemies can result in you giving them an immediate sex-grapple KO. This is nice because you can also skip the sexy fun times of the grapple if you're in a hurry, which you might be if you have to repeat any difficult sections and just want to clear the easy opponents. The game isn't super hard, but there's a couple spots here and there that could give players a slight struggle.
On that note, my only combat design complaint is the bat-winged, black-haired witch that casts triplet black-hole ranged attacks. I'm not quite sure what you're supposed to do when fighting these gals, so I usually just hit them once, wait a moment for them to reset from it, then hit them once again, slowly, ad infinitum. Sometimes they repel me and I have to reset, but this slow method is the best thing I could figure out to avoid their teleport hurting me or their triplet attack somehow insta-killing me, and I still found myself often running back to a previously defeated enemy to recover my health.
Oh, yeah, I guess I should have mentioned that the sex animations heal your character, so there's plenty of incentive to get some mileage on your futa's D.
The pixel animations themselves are all fairly good. I have one complaint about the animations, which is that the cumming sequence produces random white pixels shooting out around the characters, like Sonic the Hedgehog's rings if he was a serial sex offender. It would have been nice if cumming was part of each animation rather than a separately generated component, but that may also be part of the secret as to why the game has so many animations in it already. Almost every enemy, sans bosses, have two animations depending on if you want to be the sub or the dom, which is an option you're allowed to choose before engaging in the sex. There are also defeat animations for bosses, although I didn't really bother with watching those save for the first one when I lost to the djinn. On that note, don't bother trying to charm bosses. You'll need to use the charm button on one boss, but you'll probably recognize when that moment comes, so just wail on all the others until that point.
There are also occasional flashes of CG stills made for certain animations, although to some extent it appears the selection for which animations get these or not were made randomly. As the game is not finished at the time of reviewing, this may be addressed later, but the gallery menu does not seem to give any room for many more CGs. It's honestly no skin off my bones -- the CG stills themselves are not exactly superbly drawn, and while they did give a nice perspective shift on animations soon to be watched, there are a couple that could do with a retouch to make limbs proportionally scaled or other parts appropriately shaped.
All in all, it's a nice game with enjoyable sex animations. It won't blow your mind, but give it a shot, and you might find it a pleasant wank.