What does “to place” in the first sentence mean? Do I need to hold the cube until it touches the ground, or can I drop it? If DIF can be dropped, then can it be thrown at a distance (e.g. using rules for improvised weapons, PH 148)?
D&D uses common English. So it's literally what the word means.
http://www.oxfordlearnersdictionaries.com/definition/english/place_2
in position
[transitive] place something + adv./prep. to put something in a particular place, especially when you do it carefully or deliberately
That word was very likely deliberately chosen so that you cannot throw the tower. Because it's a portable home, not a weapon.
Dropping may or may not qualify. That's up to the DM.
What is meant by “the ground”? Must it be literally the ground, or are other surfaces (e.g. stone or sand) also suitable? Must it be only natural surfaces, or artifical ones work too? For instance, can I activate DIF on stage in the theater as a part of a performance (assuming there is enough space on the stage and it is strong enough)? If not, why not? If yes, then can I activate the fortress on a house’s roof? On a balcony? On a moving cart? On a huge mount’s back? On a ship’s deck in the open sea?
Again, plain English:
http://www.oxfordlearnersdictionaries.com/definition/english/ground_1?q=ground
surface of earth
(also the ground) [uncountable] the solid surface of the earth
Rock, sand, gravel, etc. Anything that's on the planet. Arguably the ground floor of a building or basement. Likely not a ship or the roof (assuming a roof could support its weight).
Something like a theatre stage or the like may or may not qualify. Or even a wooden floor on ground level. That's up to the DM.
What happens when I try to activate DIF if there is not enough space for it? E.g., inside a small room, or a cavern, or a narrow dungeon’s passage? Imagine a canyon 10 feet wide, with rocky cliffs on both sides; you put the cube onto the ground in the middle of the canyon and speak the command word—what happens next?
I'd rule it springs up and does damage to the room/ rock/ dungeon as if it were a creature in the area. If the damage is more than the surrounding area can withstand, the tower smashes through and becomes a tower (but likely also suffers that damage to itself).
If the surrounding area is not destroyed, then one of two things happen. If the tower was already severely damaged, it takes the full damage and shatters. The canyon wins. If taking the damage would not destroy the tower it simply fails to work and shrinks back down (after smashing itself and everything else).
What happens with the fortress if a person activates it and then dies? Can anyone else speak the command to open fortress’s door or to dismiss it? Or will the fortress remain in place forever?
It remains until someone speaks the command word and it shrinks. So it'd be possible for a tower in the middle of nowhere to secretly be a DIF where the owner died...
The front door opens with a command as a bonus action; I suppose that it can be closed like any nonmagical door, but the question is how is it locked? Is it magically locked automatically when closed? Can everyone open it from inside, or only the person who activated the fortress can?
That's in the text. It only opens at your command. It doesn't specify inside or outside, so it's both. And it pretty explictly mentions you can't open it via
knock, so it's pretty darn magically sealed.
What the text does not say is if the user has to speak the command that opens the door, or if anyone can speak it.
On the one hand, it's pretty useless if anyone can overhear the word being spoken and open the door. On the other hand, it'd be a pain if the door was locked and there was something left inside, making the tower unable to be shrunken.
The DIF doesn't require attunement, so it's probably not just the person who activated it. "You" is just the user, and the box can be passed around. So it makes sense that anyone who uses the code word can get in. So whisper.
That about the trapdoor leading to the battlement? The DIF’s description tells nothing about its magical properties, locking mechanism, or special actions to open/close/lock it. The description says that “The roof, the door, and the walls each have 100 hit points”, but only one door is mentioned here, and I guess it is the front door. So, if a trapdoor is an ordinary door (though made of adamantine), then what can prevent enemies to climb up (or to levitate) to the battlement, knock this door and get inside?
That is a silly omission. Both doors are probably treated the same, with the same hp and ability to seal themselves, because the alternative is silly.
f the enemies are outside of DIF, which attacks can they make against creatures that are inside? Can the inhabitans of the fortress be considered as having three-quarters cover for being behind arrow slits (PH 196, DMG 251)? Can they or DIF’s inner space be targeted with spells? If not, why not?
Line of sight does not change.
As per the PHB rules on cover, being behind an arrow slit gives you 3/4 cover. Which means they can be targeted by spells.
Unless there are blind spots (which there would almost certainly be). At that point they have total cover and cannot be targeted.
It seems that the fortress can be used, for instance, to cut a firebreak or a road in the woods, with the efficiency of 20 feet/round. But when I imagine such a procedure, I feel uneasy because it looks more like Monty Python’s sketches than like heroic fantasy.
This is only a problem if the party decides to do so. If the party decides not to smash a road through a forest for no reason, then it doesn't happen. It's a theoretical problem.
Sure, it can be used to smash down trees. Doing 10d10 damage to the trees every other round and an average of 5d10 damage to itself. (Resistance). After the fifth time (100 feet or so) the tower will take more damage than it has hit points and break.
On the other hand, I don’t see anything in the rules that could prevent this application. This is not a question, but rather a final lament that D&D rules’ imperfection leads to abuses and quarrels between players and the DM.
It leads to quarrels only if the players try to abuse common language. It's robbing the bank in Monopoly. ("The rules don't say you
can't rob the bank!") And then arguing when the DM tells them that, no, their ridiculous plan doesn't work.
It's not the rules leading to the quarrel, it's the player's attitude and their reaction to being told "no".
D&D is a cooperative, social game. It works entirely when there is a social contract between the players and the DM. The DM agrees not to abuse the rules and Rule 0 unfairly, and the players agree not to disrupt the game needlessly. Abusing and exploiting a magic item is very much disrupting the game.
If your DM was nice enough to give you a fun and interesting magical item, like a
Daern’s instant fortress, and you abuse the item, then all it does is teach the DM you cannot have fun magic items without trying to disrupt the game. Welcome to the fun world of +1 longswords.
