I don't remember where the castle plan I used came from, though I vaguely remember my husband printing it off the net after I couldn't find a plan that pleased me after digging through our reference books on castles. It doesn't have a website address printed on it. However, it was probably some place like this:
http://www.castles.org/architecture/
If your google fu isn't up to the task, my perrennial advice is: Head to the library. You will, if nothing else, be able to find generic floorplans of the standard types of castles - motte and bailey, timber, concentric, keep, etc.
You can also read actual escape narratives for inspiration. Every escape has two stages: First, exiting the locus of imprisonment (which may be a building, a complex, a distinct geographical feature, or even a country) and second, traversing the terrain to a place of security where active pursuit is no longer to be anticipated. You will easily find accounts of RAF and American escapes from German POW camps during WWII and survival stories from the same war's Pacific theater. Consider also American slave escape narratives, Cold War stories of escape from the Eastern Bloc and Cuba, and of course stories of escape from high-security prisons and penal colonies such as Botany Bay, Alcatraz, and Devil's Island.
Once you know who they are and have figured out why they're imprisoned, much else will fall into place. In my case, they insisted on not only breaking out of the prison but on breaking out of the town (and downtown San Antonio makes an excellent medieval city) with the children of the paladin's mentor and across country to the mentor's estate, where they finally felt safe enough to draw breath and look around them. If I hadn't had a day job (and therefore insufficient time for my style of DMing if I also wanted to write any publishable work), we could have kept going from there, because the political situation was already in place and the players had developed loyalties and causes during the course of the game that could have fueled a good strong intrigue campaign. The bad guys had no notion of the PCs, who were just a handful of civilians caught up in the mess of events that triggered the civil war, but the choices they made during the escape affected larger events and would soon have brought them into a prominent place on the national stage in an organic way with no guidance from me.
Part of the fun of an escape is the exercise of ingenuity in beating the precautions taken against is. If they can figure out how to get at the keys directly, great. If they come up with a way to make lockpicks out of innocuous pieces of clothing or bedding, even better - let them justify their attempt, set a DC for an appropriate skill (or set of skills) and make die rolls. My players figured out that sand was used in the mortar holding their cells together and managed to get the material component for a sleep spell out of it. Since one of them was a sorcerer, and one of their first moves on getting out of their own cells was to rescue some wizards they'd overheard being tortured, this was of great utility to them, and they took great satisfaction in having come up with it.
Flexibility and rewards for ingenuity - always good; essential in a game of this sort. They want to outsmart you and they want to work at it!
I didn't realize it was trite. 
