The following is the design of a unique magic item I created for use in my ADnD game. It is currently in the possession of the party's thief. For those wanting details about the Keep, read on; others may skip to the images.
The Portable Keep
In its stored form, the keep resembles an one inch clear, crystal cube with smoothly polished edges. At the center of one side of the cube is a diamond-shaped turquoise inlay. If detected for, the cube displays a slight aura of Enchantment/Alteration magic.
When placed on a solid surface and the command words spoken, the cube instantly forms into a keep (80'x90'x15') in a similar manner as the Leomund's Secure Shelter spell. The keep will be constructed of material that is common in the area (timber, stone, or sod) and lasts for up to 1d4+1 hours +1 hour per level of its creator. When its duration expires, all foreign objects within the keep are harmlessly expelled, and the keep and the materials that were altered to create it transform back to their original state.
Likened to the structure produced by a Leomund's Hidden Shelter spell, the keep is perfectly camouflaged to blend in with whatever terrain or surroundings are appropriate. It may appear as a keep-sized boulder in rocky or mountainous areas, a sand dune, a deadfall, a small grassy knoll, or even a mighty tree. Its enchantment also conceals all telltale signs of habitation, including any smoke, light, or sound coming from within the keep. Creatures or characters who are exceptionally well tuned to their surroundings may attempt a saving throw vs. spell to spot the hidden keep if they pass within 30 feet; all other creatures cannot find the hidden refuge without the aid of true sight or similar means.
Regardless of its material composition, the keep is as strong as normal stone, resisting flames and fire as such, and is secure against winds of up to 100 miles per hour. The door, shuttered windows, chimney and secret trap door to the roof (not depicted in the illustration but its location is indicated by the ladder) are wizard locked. The outer doors are protected by separate alarm spells (one sounds like a chime and the other a gong, as determined upon creation of the keep). An unseen servant is conjured to provide service to the keep's creator.
This item cannot be used in a manner to destroy fabricated objects such as existing buildings and structures, and sufficient space is required to accommodate the keep. The transformation only affects non-magical, naturally occurring material. Sentient life forms are moved aside and are otherwise unharmed. The magic of the keep can be invoked once every 20 hours.
Here are two illustrations of the Portable Keep, the general layout and specifications that were submitted to the enchanter and the actual keep as created by the cube.