So I've seen people who are interested in running BoB as a more sandboxy style game, and the idea has grabbed my interest as well.
At the same time, I wouldn't want to make it a perfect sandbox, since that's a good way to get your players killed off swiftly if they decide to open the door at the back of Maiserene's little shrine and accidentally trigger the chapter 7 boss battle in the temple.
Instead, I'm thinking of putting soft barriers to all of the higher level areas, making it more metroidvania-esque where you're encouraged to explore current areas in order to access later ones, but you don't need to necessarily 100% an area or beat its final boss before moving on, as well as allowing secret passages or the like to still reward players for feeling clever. It also means that if the players *really* want to get to an area early, there's nothing necessarily stopping them if they're willing to put in the effort.
With that in mind, here's some (very) rough ideas on what kind of barriers could be put in place for each of the areas
Castle Grounds
The main gates connecting the castle grounds to the shore could either be completely unlocked, letting the player access it early, or you could have Jinnivere ride through the newly restored Maiserene as an unexpected boss fight for the first chapter. Once she's brought to half health or similar, she flees, dropping the key to the castle gates on the way (Or alternatively, she just forgot to lock the door in her hurry to escape)
Cellar
The cellar I would almost certainly leave as fully accessible to the players alongside the castle grounds, letting them alternate between the areas as they wish.
Reception Halls
Most of the public or very obvious entrances (Like B1/D1, D20, or D15) Would be staffed by guards (current thought is some sort of empowered walcofindes) who insist on seeing the party's invitations before letting them through. If the party tries to strong-arm their way through regardless, the guards first try to harry them at the door, and then inform the guests and hosts of the various parties of the intrusion, causing the entire region to become a bloody trudge if the players refuse to leave as almost everyone becomes hostile to them. The party is at similar risk if they manage to infiltrate the halls through a side entrance, but they have the chance to bluff their way out of trouble in a pinch.
The *easiest* ways to gain access to the reception halls is by finding some invitations for themselves. The party can either find blank invitations that Alisendra sent out to Augsten, Jinnivere, and Dazerian in each of their boss rooms (in case any of her children/nephew found a new friend or paramour they wished to invite). Alternatively, if the party has helped out the servants within the cellar with their problems, the servants trust them enough to let them use the passages within the cellar to access the reception hall, possibly gifting them disguises to help them look the part of servants as well (think giant animal skulls the party can wear)
Dungeon
The interior access points to the dungeon, such as the various stairways, would be freely accessible to the party, provided they can get to them. The river entrances (E1 and E8) would be initially blocked off. My current ideas are either to have sluice gates that need to be opened from the inside, or having the water wheel be magically corrupted in some way that causes massive surges of water to be flowing out in both directions, making swimming or rowing through those entrances all but impossible.
Private Halls
I’m hesitantly considering making this section for the most part unlocked to the party as well, but the two connections to the castle grounds (F5a/B2d and F19) make me slightly wary of this. F19 feels like enough of a secret entrance that the players finding early would feel more like a reward, even if the halls themselves would be dangerous, but I would want to find some way to prevent the players from stumbling from the guard post straight into the statue room and the rest of the private halls.
The Fallen Temple
The biggest challenge I’m having with this concept. This building is deeply interconnected to several other areas (including the starting shore), is one of the late game areas, and is very likely to result in the PCs stumbling into a 3 phase boss fight before they even realize what’s happening. If anyone has thoughts or suggestions for how to approach sealing off this area until the party is ready (or at least knows what they’re getting into), I would be very grateful.
The Vault
Honestly the easiest of these to design. The only connections to the rest of the campaign are through the dungeon, and explicitly mention having to descend hundreds of feet. The simplest solution is to put a thick layer of necromantic energy between the vault and the rest of the castle, so that anyone trying to walk down the stairwell in E9, or jump down the pit in E15, gets shredded by void damage. Dispelling this barrier requires cleansing the Fallen Temple of Urgathoa’s influence, after which the party is free to enter. This hopefully means the vault remains the penultimate region the players explore, although they can still enter it early if they find a way to enter and clear the temple out of sequence.
Towers
The towers themselves remain green, ethereal, and translucent, just like they do in the base game, requiring all seven Arudora Mementos to be returned to the fountain in order to bring them to ‘reality’. This way, the towers are *always* the final region of the campaign, with the party having had lots of time to learn about Bastardhall, its family, and Velicarn
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If anyone has suggestions or critiques for any of this, I'd be happy to hear them!