On a hill near the characters' home village once stood a proud castle. About forty years ago a mage resident there summoned up something he couldn't handle, and it pretty much trashed the place. The castle consists now of the ruins of the outer towers and gatehouse, about twelve towers in all, only a couple of which have even part of a roof, six inner towers (including the inner gatehouse) most of which are in very much disrepair, and the inner keep, which is mostly collapsed. Most of the castle walls are also torn down, and the moat is overgrown as well. Under the main keep is a cellar (about three rooms worth.) All of the wooden buildings, interior wood etc. was burned. The place is rumored to be haunted, about twenty years ago old Fred went there and never was the same since.
The players recently found out that the guy that built the castle had placed a mcguffin under the floor stones in each of the towers, and a large one under the keep. (The mcguffin is some sort of enchanted jewel that was supposed to keep the castle from harm or something. In practice, any enchantment has long worn off, but the jewels should be worth whatever is an appropriate amount in your campaign.) The players are the only people (that they know of) with this information, perhaps they found it in a letter used as a bookmark in an old book.
You should stock the castle mostly with animal, or animal-like monsters. Perhaps one tower is home to a couple of giant beetles, another has some feral cats, another has some snakes. A group of brigands that operates in this neighborhood uses one of the more intact towers as a camp, perhaps they have hidden some treasure under it, perhaps several of them are there. An old crone lives in one of the towers, free rent you see. She makes healing poultices (herbal gunk etc.) for the brigands in return for food. Treat her as a second level MU with a charm person spell. You might, if you like, put a more "real" monster in the main keep, perhaps some sort of sentinel creature (ex. a water weird, one of the really minor devils etc.).
Should the party go home before clearing out the tower, feel free to replace any slain monsters with others, especially if some time has elapsed. For instance, now that the large snake has left, a weasel family has made their home in the moat. The brigands will not always be there, sometimes there may be as many as (more than the party can handle) planning a raid somewhere. Be sure to indicate signs of some of the animals, things like droppings, meal remains, shed carapaces etc. The brigands are not all that neat, there might be signs that they are around such as the tower that they use as an outhouse, a pile of cow bones, a copper penny with a recent date, a torn but unrotted rag etc.
Wandering monsters. Write up a wandering monster chart. Some of the entries should be true wandering things such as passing birds, cows etc. Most of them should be the inhabitants of the towers.