DM's Note: Daern's Instant Fortress - A Complete User's Guide

Last gaming session, the characters discovered the rare magical item, Daern's Instant Fortress in a white dragon's hoard. The player's jaws dropped as I read the description:

You can use an action to place this 1-inch metal cube on the ground and speak its command word. The cube rapidly grows into a fortress that remains until you use an action to speak the command word that dismisses it, which works only if the fortress is empty. 

The fortress is a square tower, 20 feet on a side and 30 feet high, with arrows slits on all sides and a battlement atop it. Its interior is divided into two floors, with a ladder running along one wall to connect them. The ladder ends at a trapdoor leading to the roof. When activated, the tower has a small door on the side facing you. The door opens only at your command, which you can speak as a bonus action. It is immune to the knock spell and similar magic, such as that of a chime of opening.

Each creature in the area where the fortress appears must make a DC 15 Dexterity saving throw, taking 10d10 bludgeoning damage on a failed save, or half as much damage on a successful one. In either case, the creature is pushed to an unoccupied space outside but next to the fortress. Objects in the area that aren't being worn or carried take this damaged and are pushed automatically.

The tower is made of adamantine, and its magic prevents it from being tipped over. The roof, the door, and the walls each have 100 hit points, immunity to damage from nonmagical weapons excluding siege weapons, and resistance to all other damage. Only a wish spell can repair the fortress (this use of the spell counts as replicating a spell of 8th level or lower). Each casting of wish causes the roof, the door, or one wall or regain 50 hit points.

 Now, that might seem pretty straight forward... but I think it leaves a lot of room for elaboration, particularly since this magical item is incredibly powerful. See my DM rulings below:

• How does placing the cube work and how does it enlarge? The cube must be placed on solid ground in an adjacent square to the possessor. When the command word is spoken, the cube will grow to full height instantly, during the activator's turn. It enlarges in a predictable pattern: The door faces the activator in the space where the shrunken tower was activated. The tower extends 5' to the left of the door, 10' to the right of the door, and the far wall of the tower is 20' away from the door. If unmovable barriers (such as a cave ceiling or walls) would prevent the full growth of the fortress, it does not enlarge at all.  
• Who is the "you" referenced in the description? Attunement isn't necessary to operate this magical item. But clearly the description references a "you" as the sole operator of the fortress. It wouldn't do much good to have an unbreakable door if anyone who overhears the door's command word could open it. The "you" referenced in the description is the character that initially activated the fortress by using an action to speak it's command word. As long as the tower is enlarged, the activator remains the only person able to control the doors and shrink the tower.
• Does the fortress need to be 100% empty for it to shrink? Yes. No living or nonliving objects can be inside of it... but we're not going to go crazy with this idea, either. If a mosquito flies in one of the arrow slits and stays inside the fortress will still shrink. 
• Who can open and close the door and the trap door? The description clearly states that ONLY the activator can open the door by speaking the door's command word using a bonus action. This also applies to the trap door in the roof as well. Speaking the command word is necessary to open either door when entering and exiting. Anyone can close the doors (both of which open out, by the way).
• Arrow slits? How many? Where? The fortress has eight arrow slits - two centrally located on each side and only on the second floor. 
• How does firing from an arrow slot work? Firing through an arrow slot restricts the angles the attacker can see targets to a 90^o cone shape. The attacker cannot fire on targets directly adjacent to the fortress.
• Is there a defensive bonus from arrow slots? Enemies outside the tower can attempt to attack through an arrow slit. Arrow slits provide 3/4 cover, granting +5 to AC and Dexterity saving throws. Spells that rely on line of sight can also be cast through an arrow slit.
• What is the defensive bonus from crenellations? Within each 5' square a character can decide if they are fully behind the merlons (the solid portion) or the crenels (the gap) at any point in their turn. Crenels gives 1/2 cover from attacks outside the tower which gives +2 bonus to AC and Dexterity saving throws. Merlons grant total cover from attacks outside the tower, preventing the character behind a merlon from being targeted with attacks.
• How does the ladder work? Climbing the ladder requires no roll but every 5' counts as 10' of movement.  
• How can the tower be damaged? The description notes that the tower has immunity to "damage from nonmagical weapons." The tower's resistance to all other damage is applied to magical weapons, siege weapons, spells, breath weapons, spell-like effects or other non-weapon attacks.