Located to the east of the Prosary Midlands, the Towers of Volturn are the masterpiece of a mysterious and fabled wizard Volturn, a legendary wizard of the Magocracy of Gremos. They are considered a wonder of the world, even though very few have actually seen them.
The Towers are found at the center of a circle of approximately a mile of mirror shined obsidian, which is perfectly flat, and appears to be undamaged. There are three towers located on the ground in the center of the obsidian flats. These three towers form the base for a fourth tower, which appears to be completely suspended in mid air, approximately 200 feet above the three base towers. The 3 base towers are falling somewhat into disrepair, and one has partially collapsed, but the fourth tower hangs in midair, suspended in mid-air by unseen forces, and is a glorious monument of marble and obsidian, and has not suffered the ravages of time. It is rumored that Volturn himself is in this fourth tower, living an unnaturally long life of seclusion in deep study of the arcane arts. The towers are rumored to have existed for more than two millennia, but nobody seems to know their exact age.
There are obvious entrances to the bottom towers, even the one that is somewhat ramshackle and dilapidated in appearance, but there doesn’t seem to be a clear way for a person to get into that fourth tower suspended in mid air. There are no doors, no windows, and it appears to be little more than an obelisk in the sky. Strangely, the magically hanging tower appears to have source of light, perhaps candles, fires, torches or something more arcane, coming straight out of the rock. It is clear that the magical tower is abuzz with activity, even if there is no way to get into that tower that is immediately apparent.
Nothing ever seems to go into the towers, and nobody ever seems to leave, but people have gone into the base towers and returned to tell stories of strange and wondrous devices and creatures inside the towers, but as of yet, nobody has seemed to find a way into that mysterious fourth tower.
Curiosity Killed The Adventurer: How the heck do you get into that fourth tower? Flying up to it is impossible, within ten feet of the tower, the flyer is teleported to the other side of the obelisk along their path. This makes entry to the tower a difficult task.
Planar Travel: Entry to the fourth tower can only be gained by collecting magical keys, keys that can only be found in extra-dimensional spaces that are filled with a variety of guards and traps, often magical in nature. The entrances to those extra-planar spaces are scattered throughout the towers. One could expand this to as many extra-planar dimensions will be reasonable for whatever the towers are used for(campaign wise), and they can literally be of any scale desired. Dungeon crawls, wildernesses, castles, ruins, any terrain, any idea, if you can’t think of a way to realistically include some cool idea you have, make it an extraplanar space shooting off from the tower. The reason then becomes "because Volturn thought it would be cool to place a key in this place!"
Rebuild the Towers: For extra difficulty, the final key could be located in a section of the tower that has collapsed, and must be rebuilt at great expense. Maybe they must hire sages and difficult to find engineers(possibly dead engineers), speak with the dead to get clues as to the layout, maybe they need to rebuild the tower as the party wizard researches a spell to re-open the gate.
Volturn is the Key: Why would adventurers go to all of this trouble? Maybe he is the final living heir to a throne, and must be contacted lest the line be finished. The bad guys are seeking to end the line, and only a select few know about Volturn’s status as rightful heir. He is, of course, completely ignorant of his status, and wonders why all sorts of bad guys are coming to visit him, attempting to kill him. After all, all he wants is to make strange and unique golems and research interesting spells. He’s a generally good guy, though, and would be happy to continue the line, as long as he’s able to return to his research when he’s done the deed of providing a new heir in the line. With the bad guys in such obvious competition with the party, their paths are sure to cross, and their goals will be in obvious direct conflict.
This was a simple idea cooked up to satisfy the need for strange and wondrous things to just "be there" in the world, without explaination, rhyme or reason. It provides for an excellent focal point to adventures, however, given that there are plenty of varied places to go that are all located within the towers themselves. Place them close to your PC’s home city, and you will NEVER run out of adventures to run, in the extra planar spaces provided by the strange history of the towers. It also comes complete with a built in campaign goal, one that could be as long or as short as you deem appropriate.