This is a cracking little adventure that has, in various forms, been around for a long time... it first came out for D&D 3e in 2001 (and I still have a copy tucked away somewhere), yet it has grown and developed along with the setting for which it has been written.
The introduction is a short story of a retired knight being granted what he thinks is a peaceful castle in which to settle and see out his retirement. Naturally (for where would the adventure be?) it's not quite as quiet a backwater as he was hoping for, as a nasty spate of murders plague his village and, as the local lord, it's incumbent on him to do something about it. Enter the characters...
The background, and full details of what is really going on, is provided for the Castle Keeper (GM) and it is up to the characters to make head or tail of things and pick their way through numerous red herrings - some quite dangerous in themselves - and get to the root of the problem, then deal with it. Unusually, and helpfully, there is a good measure of advice for the Keeper to aid him in keeping track of everything whilst giving the characters a fighting chance of discovering the truth of the matter. Mysteries can be quite hard to handle, either the characters miss essential clues and never solve them, or the GM has to almost spoon-feed them to the correct conclusion. With care, and this good advice ringing in your ears, you ought to be able to avoid both pitfalls. There's a lot of detail to get your head around, though, and this adventure will repay careful preparation.
There are frequent nice touches, such as a travelling circus which has passed through Capendu, the main site of the adventure, and is performing on the characters' way there. Loads going on, almost an adventure in itself, and conveying the impression that life in this alternate reality progresses quite well irrespective of the characters, enhancing the feeling of this being a living, breathing world that they inhabit along with lots of other folk. Other things happen on their way there too, some may have significance as far as the adventure is concerned, others may not. Just why the characters are heading that way is left up to you, and them. The old knight, realising that he cannot cope with a major murder investigation or growing rumours of monsters and bandits in the area, might have sent for help, or they may be just passing this way about their own business and all this appears just to be you making the journey a bit more interesting...
Throughout there is a wealth of atmospheric description and well-developed NPCs to make the whole place come alive. Make the most of it. Nothing is quite what it seems, and there is ample opportunity to build a sense of suspense, of mystery, even of fear (remember, this is aimed at low-level characters).
This is what I would describe as a mature adventure: well-crafted and embedded solidly in the setting. There's scope to carry on with other activities here, and further adventures to be had connecting in other published material or your own - yet whilst rooted in the Aihrde setting it will transplant readily to a home-brew or other published campaign world if preferred. There's plenty of challenge, thought and combat skill will both be required to bring this to a resolution. The core story may have been around a while, but this latest incarnation is well worth incorporating into your game.