The game is very difficult, but provides infinite quicksaves and has very tight controls; as a bonus Marie's current level of nudity is reflected in cutscenes and the ending, so you're rewarded for playing dangerously.
Similar to The Legend of Zelda by dungeon and sword-play.
The sequel is more of the same, with a more convenient map and animations in some scenes. There's a limit to how long you can maintain the pocket world that serves as your hunting grounds, and losing fights or letting your girls escape depletes it.
The idea is to get as many of them as you can before time runs out.
Hentai is traditionally a Japanese art form that uses the anime style of drawing to create pornographic cartoons.