Encounter Building Help: Assulting a Keep

[CleanDeceit]

I have an encounter coming up in the Shadowfell.
Players are going to attempt to take over a Keep, first planning to scout it out before they kick in the door.
My question is: how should I design the infrastructure of the keep, and what environmental details should I include, assuming that afterward, the PCs will want to maintain the Keep as their new base of operations?
I was thinking it might be more fun if there was an environmental objective… Something that prevents the players from just straight up killing the bad guys before they own the place… Maybe some kind of magical defense they need to disable… Ideas?

More about the keep: It exists in the Shattered Isles of the City of Midnight. It is not a full fledged castel capable of withstanding siege equipment, but should be defensible from ground assaults. It was once the home of a noble family who rebelled against the crown but they lost a great battle and were wiped out. The keep is currently occupied by a band of slavers (mixed Orc and Ogres) led by an Ogre Magi who I have plans for as a recurring villain.

Thanks!

[TheYell]

Im on my phone and cant post links but it sounds like a Norman fortified manor is what you want. if i remember right it is a stiff stone house with a tower and a single walled yard. if you google something like that you should be able to get simple floor plans.

[CleanDeceit]

sounds like a Norman fortified manor

ah, yeah... that is intresting
found this gem http://3.bp.blogspot.com/-IB_8d73mtfg/Uqu55JmNgjI/AAAAAAAAD5c/EvIYgNqQghg/s1600/1000889_476624885752190_199612577_n.jpg

but I am still left wondering what environmental objectives to include.

[TheYell]

Well, it was run by slavers...maybe they didn't honor the dead, just burnt them and dropped the bones down a dry well and cast magics to control the avenging undead...but they're gone! So your party keeps seeing ghosts and wights but has to search the place to find out why and then do something to make right the horrible wrongs.

[Bulhakov]

If there are slaveRs, there should be slaves. Maybe plan something with them?
An idea I've used before when my players were staging a mutiny on a slave ship - all galley rowers and slaves had a tattoo that was a magic equivalent of a slave collar (it burned and caused extreme paralyzing pain when someone shouted an activation word or if the slave strayed too far away from a totem belonging to the main slave master).
My players fund out that they could "hack" the magic tattoo and instead of pain it would provide a boost similar to a barbarian's rage (enough for the strongest prisoners to break shackles), but it required first stealing some magical paint from the main slave master, then smuggling it to a wizard undercover as one of the slaves, careful application of the paint to the tattoos (the wizard could only do it to 2-3 persons per night). Finally, when the mutiny broke out all the prisoners could provide significant help. Plus afterwards there was a plot hook of what to do with the freed slaves (in my case they wanted to turn the slave ship into a pirate ship).

[Dr paradox]

In the Shadowfell?

Here's an idea for having the villain be present, but not killable before they take the keep.

He's drunk a magical admixture and sealed himself in a specially prepared sarcophagus, entering a deathlike sleep that lets him "haunt" the keep while also not aging. When he needs to go out on the countryside and do business, his servants come and wake him up. You can have his spectral form haunt the walls with all kinds of powers, including the ability to return not long after he's killed. If the keep falls, then he can go into hiding, doing his best to drive out the intruders while hiding the location of his physical body.

[Traab]

Im not sure if this is something that exists in game at all, but how about what amounts to a ward around the keep. It has two main effects. Everyone keyed to the ward gets a +1/2 or whatever to all their rolls. Anyone NOT keyed in gets the opposite. Think of it as a bad luck ward for invaders. So they have to find the keystone and key themselves to it or else they will always be fighting at a disadvantage while inside. You can fiddle with the numbers to make sure that its not IMPOSSIBLE to take the keep without it, but still a lot harder than it should be. Or flat out make it so they cant beat much of anything inside without either disabling or taking over the ward for themselves.