Using Daern’s Instant Fortress as a portable meteor

First, some context: Some time ago I played a 12th Level Bard in an online oneshot that started us out with a choice of X magic items of Y rarity. Amongst my picks was Daern’s Instant Fortress, which was used under the assumption that, once dismissed the Fortress turns into a small cube, ready to inflate again. Near the end of the session there came a point where we made our way down a very wide circular stairwell that broadened as we descended and seemed to extend downward for several hundred feet. Down below, in the relative center was a gargantuan (4×4) big bad.

The second we were made aware of this I said the following (roughly summarized):

“I want to run back up the stairs, palming Daern’s and a Plate. Once I’m at a level that has a diameter of roughly 60 feet I want to summon my mage hand, put a plate from my mess kit (I had an explorer’s pack with me) on it and place Daern’s Instant Fortress on top of it. I command the hand to fly as close to the center of the stairwell. Once it’s there I utter Daern’s command word.”

Now, obviously the desired outcome of this would be that the big bad down below would be caught off guard and crushed by what must be several tons of enchanted adamantite. I didn’t mind that a drop from that height may destroy the item (I consider a falling Fortress a siege weapon upon itself and whatever it strikes). Other party members already used their magic items in wacky ways, so I didn’t fully catch anyone off guard with this. It did end up turning into a 10-ish minute long argument on whether Daern’s is:

A) On the “ground” while it is flat on a plate held in the air by a magic floating hand

and

B) Whether the fortress was even affected by gravity

For A) we quickly stumbled on this question on here and reasoned that for our purposes the plate as a solid surface would suffice for the cube to expand. We agreed that the item would not check if it had a 20×20 area of air around to expand onto and instead be content by having one side flat on some surface. The item’s description mentions no such thing so it shouldn’t check for that.

Regarding B) the GM and two players argued that “[…] its magic prevents it from being tipped over.[…]” in the Fortress’s description implied it was not affected by gravity and would be fixed to its location of origin.

Now, I personally believe that the magic enforcing an upright position does not keep the Fortress from being affected by gravity’s pull. There’s nothing explicitly stating that effect and as such standard “laws” should apply. Not wanting to turn this into a full out argument, I settled for that interpretation and continued playing.

We had fun throughout the final fight, but the differing opinions on the fortress left me a bit upset.

Going by what is written in the item’s description, could you drop a fully expanded Instant Fortress from a great height?