I saw that new version is out, with new scenes. Don't play it yet, but I guess that's why the vanilla version has few events that doesn't seem to effect anything (you can use cannon to blow away walls and uncover the cell holding your slave, but aside from bit of dialogue it doesn't seem to matter).
The old version is pretty straight, the gimmick is that you must figure out enemy's next attack using observe command (just check this site) then search and defeat the heroine. You can unlock different scene by defeat the heroine under specific condition (defeat her in torture room will result in you rape her while she's still unconscious instead of mind control her). It's short, but that's most likely because you're suppose to play through it few times to get all scenes...and fighting and backtracking are pretty repetive.
The visual is awesome though, the h-scene is done in manga style which is pretty nice to look.
Here's save for said rape ending.