Designing a parlor whodunit for a castle holiday in Ireland

Can't speak to properly utilizing a castle, the people or Celtic history, but my siblings and I have inherited the role of family wranglers after our late mum. Last year, both my moms and dads side of the family had the big reunions so here's my personal insight plus my GM and game design experience.

Logistically speaking:
Do not have any assumptions on the weather. It can be bone dry from now till an hour before the event; and there will be a freak summer blizzard because thats how things can go. So have a indoor backup for everything.

The other extreme is also becoming common and be ready to deal with unusual heat. Have cold packs, water bottles and basic first aid on hand because you'll likely be out in the middle of nowhere and last thing you want is an incident. And be deliberate with safety rules. Make it clear no one should be climbing trees or even playfully attacking other players.

Also sounds like you're looking for a competitive game but want to avoid natural bullying that happens in this group dynamic. I'd suggest teaming up bigger and smaller personalities and have groups focus on each other. I find that the louder ones can handle and might even enjoy butting heads and that gives space for others to shine.

As for the game. You obviously want to fully utilize this space. But there's only so much you can do in a few hours. For example, if you want to use the grounds, it's a daytime activity for an active crowd. Playing with lights and ghosts makes it a night event and you'll need to tone things down after a full day. Consider the limitations of your time as well as the strengths of your players.

Unlike a regular escape room or RP game, you've got space to work with and can go wild with physical activities. If people a sporty, have some tests of strength and dexterity. If people are more puzzle solvers, make scale a factor.

I'm just going to throw out a high level concept based on my style of design and storytelling.

Instead of solving a murder, I'd propose players try to prevent one. It's a proactive objective with a time crunch.

The game begins with a threatening note appearing, claiming that someone there had wronged the villain and they have been hiding in the castle for months, planning to exact their revenge. The authorities are on the way, but they might not make it in time so it is up to you guys to find the would-be killer. And you can even get a reward if you can correctly identify the killer, the target, the motive and the planned murder weapon. But you need to be in teams in case the killer is amongst the players.

The letter should have a very obvious clue that would lead teams to a sort of hub space. Most likely the killers hide out. Then there should be clues and notes that lead players down the path to a specific answer with hints at other answers along the way. Muddy boots lead you to the grounds. A stack of books hints at the library. Cooking utensils point at the kitchen. And so on.

From this, you have a sort of agnostic framework to build off. In terms of plot, you can plug in a modern set of circumstances like a Knives Out situation. You could find the regional history and take liberties with the characters and events for that historic story. You could even use the actual family history and learn more about that in the process. At my dads family event we had a quiz where kids needed to get certain information from family stories and they had to pry that info from the older guests. Was a lot of fun to watch.

In terms of actual games and puzzles, you can map a system where you can slot in activities and information. And you can swap things around if the situation changes. For example: the killer has burried something in the woods. And to find it, you need go follow a path of etched on trees. But if its raining, you can move it to the library and you can mark different books with a symbol sticker, leading players to the same evidence hidden in a hollow book.

More specific puzzles/games: the victim has a favourite food but serious allergy. And there happens to be several dishes prepared in the kitchen that may have been tampered with.

The killer got bored and would often sketch out the events that set off this murderous obsession. And the players will have to piece together the correct sequence from the different pages scattered around the house.

Have each persons room door marked with a subtle symbol with the victims symbol holding special significance.

If you sit on a certain chair and look out a window, you could have something conveniently framed or find an important clue by drawing a line between landmarks.

Just throw a giant keyring with hundreds of keys in the mix. Doesn't even need to do anything, but people obsess over it to a hilarious degree.

Also read up on escape room design principles. It makes for a very useful design philosophy in general.

As for the social intrigue, you can get that from the prize incentive or other elements. In a perfect world: each team should focus on one thread, come together and solve the mystery. But if there is a tempting enough prize, players can be sneaky and share incorrect info. Or even outright tamper with clues and paths for the next team to find. That's the holy grail of mystery design is where there's multiple ways to solve it that are all satisfying. But there should be enough to call out someone's BS.

You can also seed traitors into the mix. One per team. One entire team. I personally don't like that style of game but it has its appeal.

When I wrote/ran one in the past for a large group (30+) I had a core group of 6 people (including the person that will be murdered) be essentially actors for the event. These individuals were in the know of what would happen and their relationships with each other. We had the murder play out shortly after party started (lights turned off, body on the ground). For the rest of the guests they know the theme ahead of time and that they will be trying to figure out who among the remaining five individuals committed the murder.

You will want to create a matrix (for your usage) of all the suspects and columns for motive, means, opportunity, connection to murder weapon.

Everyone of the five should have multiple boxes checked and corresponding evidence/statements made by others collaborating as such. (AND evidence/statements that could collaborate unchecked boxes). The key is the only the murderer should truly check all of those boxes.

Another layer to add is red hairings to make it harder to parse for your guests. Keep the number if these low as you will need to thoughtfully weave these into the narrative. Two people who dislike each other casting needless blame on each other, etc. I tracked these with red checks on my matrix. Each red hairing should have evidence that refutes it that can be found by your thorough players.

We concluded the party with meeting where we let guests share who they thought had done it. In my experience not everyone will proffer one. We had the killer draw a "weapon" at the end on the person who guessed correctly and provided the most correctly collected evidence for the killers guilt. We then had the some of the 5 core members take down the killer.