Europe is a pretty interesting (and weird) place, even if you ignore its potentially monstrous royal bloodlines and its assorted botched 16th century alien invasions. The Rhine Valley seems to have attracted more than its own share of High Strangeness, due in some small part to the fact that the area around the powerful (and mostly navigable) river has been inhabited since before the dawn of recorded history. But we don't need to go back far to find something useful. The 13th century seems like a good time to start, but we still need a place: how about Lahneck Castle?
Contesting Crowns
The earliest portions of Lahneck Castle -- Burg Lahneck to the locals -- was built on a rocky hill overlooking the convergence of the Lahn and Rhine rivers in 1226 AD by the Archbishop of Mainz (a major city further up the Rhine) to guard the strategic river-crossing, and a series of productive silver mines that had been recently discovered nearby. Over the next 70-odd years the castle (and its feudal lords) grew in prominence, ultimately playing host to King Adolph of Nassau, who was betrayed -- by the lords who had originally elected him -- and killed in battle just after leaving the castle in 1298.
Adolph's successor (conveniently, also the man who led the army that killed him) was Albert I of Habsburg, a previous rival for the throne whose major dispute with Adolf had been an argument over the ownership of the Regalia of the Holy Roman Emperors and Kings. Which, for those following along, includes the Spear of Destiny and the famous (possibly also holy) sword of Charlemagne (who, you will remember, is the guy who booted the creepily aquatic Merovingian wizard kings out of power in Europe 500 years before).
Speaking of questionable ancestry, it's a well known fact (or at least "fact") that the Habsburg dynasty (of which Albert I was a member) is a key part of the Illuminati, probably because they are actually extra-dimensional Satan-worshiping reptoids (just like England's House of Windsor). Use the ongoing struggle among fractious 13th and 14th century German nobility as a backdrop for a historical fantasy of noble knights and rightful kings combating secret reptilian anti-Paladins and corrupt, grasping princes in cliff-top fortresses in a desperate struggle for control of ancient artifacts of tremendous power, or a game of medieval intrigue and espionage as agents of the Pope (or a neighboring king) attempting to unravel a vast conspiracy against Europe, the Church, or life itself.
Last Stand of the Templars
The castle's knightly intrigues don't stop there. When Pope Clement V ordered the Knights Templar to disband at the prompting of his cousin, Philip IV of France (a secret Merovingian?), many knights of the order fled to avoid Clement's wrath. According to local legend, the last 12 members of the order took refuge from Clement's agents in the castle in 1312. The refuge proved only temporarily effective. The powerful Archbishop of Mainz, Peter of Aspelt (one of Clement's agents, and a close ally of the Habsburgs) pursued the Templars to the clifftop fortress (which, technically the Archbishop still owned), ordering a siege against the castle that lasted well into 1313.
The 12 defenders outlasted several direct assaults, eventually forcing the commander of the Archbishop's army to fly the flag of truce and offer the last living Templar a chance to leave the castle unharmed if he would reveal his order's secrets, ask for the mercy of the church, and renounce his membership in the supposedly heretical order. The bloody, weakened knight stumbled out to meet the enemy commander, apparently willing to surrender. However, when he reached the enemy commander's position on the drawbridge over the castle's dry, spear-filled moat, he started running, tackling the general and carrying both himself and the Archbishop's agent to their deaths on the spear tips.
It would not be difficult to weave the Templar narrative into a campaign of knightly struggle against the dastardly reptilian Habsburgs, apostate popes, and their degenerate allies (Warhammer Fantasy Roleplay? [K]Night's Black Agents?), but you can also use them to turn the whole thing on its head. After all, despite the brave tale told above, history doesn't uniformly cast the Templars as the good guys -- they've got as much or more conspiratorial baggage than anyone else in this mess.
Maybe your PCs are heroic warriors sponsored by the True Power of Rome hunting the false knights of a debased order attempting to hide the last of their secret treasures below the castle in the silver mine tunnels of the Taunus mountains. That evil Archbishop of Mainz? Actually one of the few true followers of God in a dark Teutonic fairy tale land of enchanted castles and crypto-Mohammedan blackguards. Or maybe it's all shades of gray? Evil Habsburgs and Corrupt Popes vs. Degenerate Kings and Heretic Knights. And your poor PCs caught in the middle. At least they don't have to worry about which ones are the bad guys.
An iron treasure chest with an elaborate series of locks.
Hauntings and Other Problems
Or maybe you want to run a game with a more modern flair. It's worth noting that the castle is also supposedly haunted by the ghost of a Scottish girl who died there in 1851. 17 year old Idilia Dubb was exploring the ruins of the castle on her own, climbing the only indifferently maintained central tower with plans to sketch the surrounding countryside when one of the central supports collapsed, dropping the girl deep into the castle dungeon. The walls were thick enough that her parents could not hear her cries when they went looking for her several hours later. Though Idilia was hurt, she remained alert, ultimately surviving for several days without food or water, using her sketchbook as a journal to record her final days. Her parents, convinced that she had simply run away, returned to Britain without her, and it was only in 1862 that Ms. Dubb's body was found.
Ms. Dubb's ghost -- fearful, starving, and angry -- makes an excellent antagonist for one-shot horror, though you could also use her story to lend a supernatural air to attempts to scare off tourist patrons in order to allow a local industrialist to make a mundane attempt to reopen the silver mines or recover hidden Templar treasures, a la Scooby Doo (perhaps using Dread, or the forthcoming Bubblegumshoe from Evil Hat).
He would have gotten away with it, too, if it weren't for you darned kids.