It's like a Bag of Holding but you can only fit one thing in it, and one type of thing. Pull the drawstring of the bag open, hold it near the castle with one hand, reach out, grab the wall and with a gentle hand smoothly tip the castle into the bag.
Do it handily and it will come off the ground like a cereal box and disappear into the bag like a snooker-ball into a hole.
But hesitate, judder, pause or flail and the mass of the castle comes back and you die in an avalanche of stone, along with anyone inside.
Once the castle is inside the bag it fits quite neatly, it looks and feels as if you were carrying around a dolls-house in a sack. If you judder or smash the bag then the castle can get damaged inside.
From the perspective of those inside it feels like the castle is now in a gloomy non-dimensional space. They can't see the fabric of the bag or get out. If you open the top to look in they see an polarised pale glow, but they don't see your giant face looking down at them.
If you dump anything 'normal sized' in the bag then it will crash into the castle. It retains its relative dimensions. If you are being nice you might want to pour corn down a chimney so they can make giant popcorn to feed themselves. If you want to be horrible you can dump a giant rat or some insects in there so the people are chased around the castle by the huge beasts.
When you tip the castle back out, any huge things inside will still be huge. So, huge man-sized popcorn and huge insects and huge rats. You can try to dick about pouring in gold coins and gems but people with a castle bag usually have more important things on their mind, plus when you tip the castle back out all the people in it will still be there (unless they starved and died, in which case they will be undead) and they will want to guard their giant gems and gold coins.
Time does pass in a castle bag, but slowly. Anyone who dies in one becomes a scale-shifting undead and these undead can climb out of the bag, obsessed with killing whoever put them in there and with taking the bag themselves. And this is what ultimately happens to many castle bags, they are left too long and the castles inside them poorly treated and they are stolen by some scale-shifting undead and taken off to some dark corner of the world where the creatures sit and play with their castle.
If you find a castle-bag that already has a castle in it then be careful because if you tip the castle out then it may be full of angry undead and weird spirits.
It could also be full of treasure though, or artefacts from whenever the castle is from, plus its still a castle that you can put wherever you want (so long as there is room), and that's useful.
Plus you could just dump the castle out somewhere and run for it and now you have an empty castle-bag and you can grab a new castle if you want to.
WHAT COUNTS AS A CASTLE?
It's best not to get too clever with your definitions. If you slip over into mansion territory on one hand, or just a hill with a fence on the other, then you will only find out if the bag will not accept the castle at the worst possible moment; just as you are about to tip it in.
The bag also only accepts built elements, so if it’s a fort on a hill 90% of the hill will be left behind. In situations with attached aqueducts and other buildings it will usually cut them off after a few metres.
Not what I was thinking of when I came up with this
but if you want to find out more about Helen Anthonys castle bags
there is an article here.
WHERE CAN I TIP THEM OUT?
The bag will do its best to integrate the castle with wherever it is tipped out, but there are limits to what it can do.
There has to be room. If there are other buildings on the site then, if you are lucky [if the DM has prepped something or doesn't mind winging it] then you might get a mad Escher/Tardis style integration of forms. If you are unlucky [if the DM is pissed with you or you roll badly] you get rubble.
If the shape of a castle is very highly integrated into its environment (like a mountain peak, a river crossing or a port) then its best to try to find somewhere similar to tip it out or you might get a bad reaction.
The bag can't generate water courses. If there is one there then the bag will try to plumb in wells (if they exist) and sewers (if they exist) but that can easily go horribly wrong. If there is no water course then the sewer backs up and the well has no water.
WHAT'S THE LEGEND?
In history there were only ever thirteen Castle-Bags.
Three made by 'Brass Beard' a short and hairy Rumplestiltskin figure in a complex ongoing situation with three Princesses, three Kings, Three Castles and the exact wording of a wish
Seven made by the Greatest Thief Ever Known, who famously used them to re-orient the military border between two powerful nations as part of the confidence scam that won him (or possibly her) a continent, the so-called 'Continent of Thieves' which does not appear on any map and which can be found only by Thieves, and which probably doesn't actually exist or if it does exist is likely a China Mieville-esque metaphor for the invisible nation of thieves that exists inside and upon our own*.
One handed over by the Devil at a crossroads.
And a pair made for two sisters 'Lady-Bird' and 'Snail-Girl' in a Steppe-Myth which turned out to be true. One bag being forgotten in the treasury of a major Kingdom for a millennia and the other found in a Steppe Tomb.
WHERE ARE THEY NOW?
Eight are held by the two most powerful adjacent nations in the world. Both put enormous resources into attaining and retaining as many bags as possible and maintain a balance of Mutually Assured Parcelling. Each Nation has four each. If one were to gain an advantage in bags then it could precipitate a war.
Three are inside each other, (you can do that, so long as each has a castle in it) with the last being held by 'Brass Beard', this being the current situation with the Princesses. Brass Beard is searching for his True Love, but he is a manipulative riddle-obsessed sociopath with poor person hygiene whose only positive qualities are his ability to make interdimensional bags. He also likes vegetables carved from jewels (lettuces from emeralds, radishes from rubies etc) and one particular song played on the piccolo, but nobody knows which one. To summon him simply sing "Brass Beard, Brass Beard, where are your Swans? The season is setting and shadows are long." three times and he will usually turn up. (You don't want this to happen).
One has been dragged off somewhere by the undead occupants, maybe to Fairyland.
One is in Hell and is used by the Devil to grab new castles to populate that land and also handed out in more crossroads situations to invariably negative effects for everyone.
WHATS THE ADVENTURE?
Someone has found out how to make the bags and is handing them out. A Castle-Stealing war has begun. Troops of Thieves called 'Fortification Men' are sneaking about between kingdoms. No castle is safe. It's geopolitical and architectural chaos. No-one knows what their defence posture is. Every nation seems vulnerable but castles could pop up in strategic places at any time. Peasants are briefly happy that the dicks in the castle have gone until the military chaos enfolds them too. A ridiculous number of Dukes, Duchesses, Princes, Princesses and Horses have all been stolen, along with all the ordinary people paid to follow them about. Knife-fights between gangs of thieves are leaving castles strewn all over the docklands. Someone recently bet a castle on the turn of a card. Confidence men on every corner promise to sell you a castle in a bag, some of them may even be telling the truth. Adventurers are being hired to steal castles, to get back stolen castles, to defend castles from being stolen, to find out what is going on, to stop anyone finding out what is going on. A dwarf calling himself 'Bran Nine-Castles' is heading for the Frontier with what he claims is the beginning of a fresh Empire. Bags are banned in many places and anyone carrying a bulky object in a bag has become a suspect. A small army of bounty hunters was held in a standoff for five days by a Bedlemite who said he would drop a castle on them. Is it Brass-Beard? Is it The Greatest Thief Ever Known? Is it the Devil? The Gygaxian locig of a replicable effect applied to a magical situation? U DECIDE!
See, I did it in one paragraph. Efficiency.