This is Blackrock Castle. Built in Cork, Ireland in the 16th Century to repel pirate attacks. Its stalwart fortifications boast several towers with walls over 2 meters thick to withstand cannon fire.
It now serves as an interactive astronomy center whose purpose is "The Search for Extreme Life in the Universe".
But let's say you want to convert this to a fantasy/game setting....
Perhaps a "mad" wizard is holed up behind the thick walls attempting to contact other worlds. He could be doing so through use of a massive telescope that sits protected within the tallest tower. Or he is gathering cosmic energy from multi-colored crystals set in the top of each tower that harness various powers. Through manipulation and calculation the wizard is channeling these latent energies into the castle courtyard, whereby the stones themselves acts as one large resonator to open portals to other dimensions.
Is this wizard truly a wizard at all, or is he a misunderstood genius hoping to reach other planes of existence? What is he hoping to accomplish in this venture - a heightened awareness for all mankind, or the destruction of the material plane by otherworldly demons?
How do the players hear of this: flashes of colorful lights in the night sky, a low hum that can only be heard by pressing one's ear to the ground, the crackle of electric charges and freak lightning strikes from a cloudless sky?
Once they know of the experiments, do they wish to stop it, or explore the mysteries with the wizard?
Depending on which they choose, do they attempt to help the nearby villagers breech the thick walls, or must they reach the parapets first to repel the attack?
How does one gain entry? Through a hidden underwater passage, a large magical gate with a series of locking runes, or perhaps an invisible force field protects the castle and someone can only pass through unarmed and unarmored, with pure intentions.
Once inside, it could be discovered that an unstable wormhole has already opened. Maybe the wizard meant well, but the trans-dimensional energies that lulled him into completing the ceremony/opening were in fact malevolent and now a demonic god hopes to reach through and obliterate life as we know it. Or instead of something coming through the gateway, our world is in peril of being sucked into the void a bit at a time. If the portal isn't closed, the very fabric of existence could unravel!
Either way, what are the consequences?