There really is not much more than conquering the keep and rescuing the princess at this point. Act V should hopefully come out in the next few months, but it's a major development that has taken ages.
If you want to see the intro for act 5 and sleep with the princess, you can donate $20 to subscribstar or buy the game on steam. Other than that, there is lots of replayability with the game as you can work towards getting all the consords and scenes.