What appears to be a snow globe is a powerful magic item which can be used once per week to create a defensive structure that can be invested by up to 80 humans or 20 humans and 20 horses.

The central courtyard is a 30 foot square surrounded by a sloping 10 foot high, 20 foot wide sand wall. The top of the wall has a 5 foot parapet protected by crenelations, giving those occupying it 75% cover, (100% if seeking cover and not fighting or looking.)

The outer wall is sloped at almost 80 degrees, and climbing it is nearly impossible because hand and footholds crumble when weight is applied. (Spider Climb works, and sand dwelling creatures with climb ability can traverse the structure normally.)

A 10 foot deep, 20 foot wide ditch surrounds the wall, except at the gate where a 5 foot wide causeway crosses the ditch. The gate is located at one of the corners of the structure, piercing a drum tower which is 20 feet in diameter and 20 feet tall. It is similarly crenelated leaving a 10 foot square at the top for use by the defenders. Access to the parapet and the drum tower top is via external stairs. Aside from the gate tunnel there is no 'inside' of the structure, although fake windows line the walls and tower. The gates themselves are just as strong as the wall and cannot be bludgeoned open.

The walls are virtually impervious to any attack. A sow, (siege engine that undermines walls,) or sappers, (miners,) will breach a wall in half the normal time, but ballistae and catapults will only do cosmetic damage. Of course, hurling burning barrels of naptha (and similar other attacks) into the structure will affect its occupants, but the castle will be unaffected.

While it is a temporary structure, over the seven days it can stand, any damage to the structure will slowly repair itself so long as the snow globe remains within the castle. At the end of seven days the snow globe can be used to rebuild the castle like new.

Rain can destroy the castle. Each hour of steady rain reduces the effectiveness of the crenelations by 15%, and while it is raining the castle cannot repair itself. When the crenelations reach 0% cover the drum tower will collapse and the outer walls will crumble. When the rain stops the castle will slowly repair itself, requiring 8 hours to rebuild collapsed walls, 16 hours to restore the drum tower, and an hour per 5% cover to repair the crenelations.

The castle cannot be erected on or over an existing structure. A road, house, barn, or even an outhousd within the footprint of the castle causes the magic to fail, using up its weekly charge. When the castle is abandoned, (the snow globe is taken more than a mile away,) it becomes a mound of sand. Over time it will wash back into the ground.