Tough boss battles are one of the main reasons that people play any game in the Souls franchise, and the same holds true for Demon's Souls. Between the Armor Spider of the Stonefang Tunnel and the Tower Knight of the Boletarian Palace, players will have their hands full with some pretty difficult boss enemies. The third of the five Archstones in the game offers a unique horror-vibe, the likes of which the Souls series has never again accomplished. Despite an emphasis on terrorizing players, the bosses found here are still just as challenging as those from other Archstones.
When determining which of the three Tower Queen Archstone bosses is the hardest, it's important to remember that players will each have different experiences. Based on what starting class and build players have decided to go with, certain fights will be easier than others, and the non-linear approach that Demon's Souls takes to its story means that these bosses could be the first or last ones that players encounter on their playthrough. Despite this, each boss does have an innate difficulty that transcends these things, so picking one that is definitively more powerful than the others is still possible if not somewhat subjective.
Prison of Hope - Fool's Idol
Most new players will probably recognize fairly quickly that the Prison of Hope should be saved for later on in a playthrough, as the powerful Mind Flayer enemies are capable of dealing heavy damage from afar and can be difficult to manage early on. Despite this, some players still wind up facing the Fool's Idol at a relatively low soul level and with little in the way of experience. Thankfully, with a little bit of strategy, this fight is actually rather easy, not requiring much of players other than patience and a methodical approach.
Naturally, players that run in guns blazing are more than likely going to get surrounded by the Fool's Idol's clones and blasted back to the Nexus, but the boss arena does a lot of work to help players here. Using the pillars as cover, players can systematically determine which of the Idols is the real one and then land a ranged attack on them to dispel the other clones. At that point she'll be open to attack for a while, allowing players to get some heavy damage in before running back to cover to repeat the process.
Upper Latria - Maneaters
When players finally enter the fog door of Upper Latria, they'll feel a sense of relief that they're almost out of that forsaken area. This relief is quickly shattered as the Maneater comes barreling down the thin walkway towards the player. Battling this creature isn't actually terribly difficult, and it seems like this is going to be another easy boss fight, and then a second health bar appears at the bottom of the screen. Frantically spinning the camera around, players see another Maneater fly in and land in the already crowded boss arena, and at that point the real fight begins.
The Maneaters boss fight is notable for being Demon's Souls' only two on one boss fight in the game. Unlike titles in the Dark Souls series where these fights happen somewhat frequently, players won't encounter this specific breed of challenge again on their playthrough. Dealing with two of these beasts is difficult enough, but the AI of the Maneaters is also somewhat buggy, causing them to spend a ton of time flying through the air and making blind charges. To make matters worse, even characters with high defensive stats or the Warding spell still can die in a single hit if they are knocked off the edge.
The Ivory Tower - Old Monk
After finally triumphing over the Maneaters, players will ascend the Ivory Tower and find themselves at the Old Monk's boss door. This fight is unique because rather than facing a giant monstrosity of some kind, players instead face a Black Phantom controlled by another player. Because of this, the fight has the potential to either be the easiest boss fight in the entire game or the most difficult that players will ever encounter. Despite being forced to wear the Monk's Head Wrappings, the invading player still has all of the stats, equipment, and abilities of their character.
On the one hand, it's entirely possible that players will enter this fog door and find a red phantom that's AFK completely defenseless. In this situation, the boss fight can be won with no effort at all, simply chaining backstabs until it's over. On the other hand, if faced with a formidable and practiced opponent, this fight can be literally unsurpassable for some. Thankfully, players always have the option of going into offline mode and fighting a computer controlled Black Phantom, resulting in a much easier and more manageable fight.
Demon's Souls - Archstone of the Tower Queen Hardest Boss
All of these bosses, even the Fool's Idol, have the potential to cause players huge problems. They each provide a unique type of challenge that isn't found anywhere else in the game, and players will certainly have varying degrees of success when taking them down. With all that said, it seems clear that among them, the Maneaters boss fight is the most difficult. The Fool's Idol is widely considered one of the easier boss battles in Souls franchise, and though the Old Monk can be challenging, players can always fight the offline version which is much easier.
Whether it's fair or not, the Maneaters are a huge wall for many players as they attempt to progress through the game. Keeping track of both beasts at the same time is very difficult, and a single hit from either of them can deal massive damage to unprotected characters. Their buggy and frustrating AI makes the fight even more difficult, and players have to run through the unpleasant Upper Latria area (and past an infamous Black Phantom Mind Flayer) every time they fail to defeat them. It's clear that the Maneaters are deserving of the honor of hardest boss in the Tower Queen Archstone.