The Mages Tower

By inscribing runes into a tower or other structure a wizard can push the structure into another plane where it exists forevermore, accessible only by a single key, anywhere the wizard may be.

Once a suitable tower or other structure has been found or built, extremely intricate runes must be carved on the outside structure. This process takes 1 week and costs 250 gold per 1000 square feet.

Once the preparations are complete, you cast the spell with a DC of 14 for a 1000 square foot structure, the DC goes up by 1 for each additional 1000 square feet of building size. Other Wizards can attempt to assist by expending a spell slot of equal level. This gives you a plus one bonus to the roll per Wizard helping.

On a failed roll, the structure takes damage as the spell fails. It takes 500 gold and a week to repair the damage before you can try again.

On a successful role, the key glows bright and the structure vanishes as it is pulled into its own pocket plane. Breathable air is the only thing the pocket plane provides; anything else must be brought in by the players.

To access the structure, the character must hold the key and cast the spell. A keyhole then appears and you are able to unlock the entrance. Once unlocked the doorway into the structure will appear. Only the person holding the key is able to hold the door open. Anyone else who tries to hold the door open is forcefully pushed out of the structure and back into the material plane. Everything in the structure stays as it would in the material plane; thus, food will rot if left. To leave, one must simply unlock the door with the key. 

The key can not be duplicated by any means. Any attempt may create what appears to be a copy, but that copy will not be able to call the keyhole or access the structure.

Do not loose your key or lock yourself out.

Casting dispel on the key or on the structure while in it will rip the building back into the material plane, causing it to appear in the original place it was before the spell was cast. DC 25