They won't let you in unless you're dressed in the Royal Guard armor. The only way to get it is to do the Aelinore quest line:
-- give her the silly hat after meeting the Duke -- sneak in to her room, watch her get choked, save her, get whipped -- talk to her lady in waiting later, who gives you the quest to rescue her from the Manse --- "Sigs are for dorks."-- my wife
You gotta tap dat duchess assssss. Then... Wait what were we talking about?
Oh, right: Then, lata on in da evenin'... wait, that's not right either.
Hmm... Ohhhh, yes, the duchess' little maid-lady will hail you, and by that I mean wait in the gardens, and tell you to save the duchess in distress. Which, I believe, is the name of the quest. She will give you the royal guard armor, so that you may pass through the front gate.
Or something like that. --- Gamer Tag: Coltonius (Pawn: Eve) "I smoke 'cause it gives me knowledge!" - Citizen of San Andreas
The quest is available right before you head out to fight the Dragon, Grigori. The Dutchess' chamber maid is standing outside the castle to the left of the stairs. --- "Why can't we all just be friends?"
That's one thing i don't like about this game. I've taken Chimeras, Drakes, Griffons, armies upon armies of Goblins and Bandits - yet I'll take punishment from Duke Has-Been.
I'm in my prime, I could and would love to take Gran Soran apart with my bear (rawr!) hands. Then watch the Duke's eyes go lifeless as I strangle the hell out of him for dealing with his crap. Hell, I'm saving the damned Kingdom. And I'm going to be whipped?
You first get into the manse by during the Duchess in Distress quest. But does anyone know if you can get back in after the quest is done. I managed to get in the front with the Royal Guard armor but can't get past he gate/grill to go deeper. Skeleton key's don't even work. Is there a key hidden away in there or something?