Barriers and Obstructions, Tier 4

Previously: Barriers and Obstructions, Tier 3

Characters: Few character abilities at Tier 4 bear directly on overcoming barriers; possibly the monk’s Empty Body feature, allowing them to cast Astral Projection; clerics with improved Divine Intervention

Spells (level 9): Astral Projection, Gate, Shapechange, True Polymorph, Wish

Magic Items (legendary): Amulet of the Planes, Cubic Gate, Plate Armor of Etherealness, Well of Many Worlds 

Soft Barriers

Demiplane + Mordenkainen’s Private Sanctum or Forbiddance. Creating a Demiplane and then warding it against extraplanar travel is about as secure as you can get within the rules of the PHB spells.

But challenges at Tier 4 (levels 17-20) should hardly be constrained by the PHB. Antagonists at this tier are ancient liches, elder dragons, demigods, and extraplanar aberrations. In addition to the dungeon defense spells detailed in Tier 3, Tier 4 opponents can bend or break the rules that would constrain conventional spellcasting. Think permanent or programmatically cast Antilife Shells and Antimagic Fields. Permanent Prismatic Walls. Spheres of Annihilation, not just as encounters or traps, but even just as navigational obstacles. Walls of Annihilation? The Maze of Annihilation?  

The important thing is that options like these are deployed in an open scenario where the characters have exploration options and ways to understand and counteract what is going on.  

Hard Barriers

None, really. At this point the characters are bending the laws of reality.