In order to break the barrier around Hyrule Castle, the only necessary flag is watching the cutscene that plays after defeating the Mighty Darknuts and exiting the Master Sword Chamber in Hyrule 3. However this comes with its own set of flags including collecting the completed Triforce in order to reach Hyrule 3. While collecting the Triforce shards early seems like it would allow us to skip completing the Earth and Wind Temples, completing the Triforce and then entering Hyrule 2 via completing FF2 automatically puts Hyrule in a Hyrule 3 and not Hyrule 2 state--overriding a cutscene that is a necessary flag for triggering the aforementioned cutscene in Hyrule 3 (see the bottom of this page for more information).
Out of the eight Triforce Shards and Charts, charts 1,3, and 5 can easily be obtained early with only Grappling Hook and Bombs (for more information on the locations and methods of obtaining all Triforce Shards and Charts, click here). Acquiring the other five charts, however, requires a more intuitive use of glitches to skip additional requirements.
In the event that a solution is discovered to the current dead end (see below) that keeps us from breaking the barrier without the true Master Sword, the Triforce would likely be collected prior to completing the ToG. Doing this, however, entails getting around the places where the hookshot, skull hammer, and power bracelets are otherwise required. This most often requires manipulation of storage and its effects:
Although it is possible to enter the Cabana on the Private Oasis without the Cabana Deed (click here for more information), all current methods require having the hookshot. However it is also possible to enter by utilizing the effects of door cancel. To do this:
Once inside you will also be required to skip the skull hammer requirements in the basement. This can be done by navigating through the first part of the maze, using a sidle to clip past the gate, and proceeding through the remainder of the maze to the chart.
The layout of Diamond Steppe Island makes it very easy to enter without needing to use the hookshot. To do this:
Alternatively you can use one of the small cyclones around the island to glide to it, and then use a wind waker dive to achieve the same effect as with chest storage.
Note: Swimming under any of the bodies of water on the island will cause you to surface. Use caution to avoid surfacing on a lower level.
Though the Savage Labyrinth can easily be completed up to the triforce chart, and can be easily reached from the outside without the hookshot using the deku leaf, the power bracelet requirement still must be skipped. To do this:
As with entering the Savage Labyrinth door cancel can also be used to bypass the large headstone blocking the entrance to Stonewatcher Island. However unlike on Outset, you cannot simply walk through the stone to reach the hole, otherwise you will get crushed. To do this:
Note: The size of Link's hitbox is far smaller when using door cancel than with chest storage. This means that if you are using the trees to reach the higher levels of the island you will have to walk towards their outer edges to avoid simply walking through them.
The obstacle of needing the hookshot can easily be overcome on Overlook Island with the aid of chest storage. To do this:
Zombie hovering and healing with the Tingle Tuner could also be used as a viable alternative to reach the pillar. However this is likely slower and not recommended.
Although it has been discovered that having the true Master Sword is not a requirement for breaking the barrier in Hyrule, and that the only flag for doing so is watching the cutscene that normally plays after defeating the darknuts and exiting the Master Sword chamber in Hyrule 3 (which, in turn, has its own set of flags), there are additional obstacles that prevent us from skipping the Earth and Wind Temples:
Q): How do you trigger the required cutscene?
A): You need four things to trigger the cutscene after the darknuts. You need to pull the Master Sword, watch the cutscene in the courtyard (the area where you enter Hyrule) that plays after Hyrule 2 where Tetra wakes up and KorL talk to you through the gossip stone, watch or at least activate (you can activate but still skip) the cutscene where the KoRL reveals Tetra as Zelda, and kill the two darknuts.
Q): So what does this mean you have to do game play wise?
A): It means we have to visit Hyrule 1 via Tower of the Gods and pull the Master Sword from the pedestal. We need to collect the Triforce to trigger the darknut fight. We also need to beat Forsaken Fortress 2 to trigger the courtyard cutscene and the cutscene with KoRL in the Master Sword chamber.
Q): What would this skip and how much time would it save?
A): If we could get this to work we would skip the Power Bracelets, Iron Boots, Earth God's Lyric, Wind God's Aria, the entire Earth Temple, and the entire Wind Temple. This would save around 40 minutes. Maybe even more.
Q): What is stopping you from using this sequence break?
A): The courtyard cutscene is a severe bottleneck unfortunately. In order to use this sequence break you need to get the Master Sword, collect the Triforce, then go complete Forsaken Fortress 2. The issue with this is that having the Triforce and entering Hyrule 2 turns it Hyrule 3. This is a problem because the courtyard cutscene does not trigger in Hyrule 3, only Hyrule 2. As mentioned before, watching this cutscene is required to trigger the later cutscene which allows us to break the barrier.
For more information, click here.