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There exists a mighty stronghold on a bountiful greater demiplane. The only way in is through a permanent planar gate on the material plane, around which has been built a great and legendary door. During peacetime, it is open and unsealed for all to pass through.

From Create Demiplane, Greater:

Portal: Your demiplane gains a permanent gate to one location on another plane, which can only be used for planar travel. This location must be very familiar to you. This gate is always open and usable from both sides, but you can secure it using normal means (such as by building a door around it).

However, a great invasion is on the horizon. They will soon arrive to siege the stronghold and attempt to break down/through/into the gate. If the army is able to enter through the gate, the stronghold will surely fall, those inside will be destroyed, and it will usher in a new age of darkness, yadda yadda yadda. Assume the invaders cannot simply teleport into the stronghold, nor can they break in through any avenue other than this gate.

The stronghold has had sufficent warning to make any preparations they wish, and they will naturally seal the gate door shut with everything they have. The inhabitants of the stronghold have access to any non-mythic spell or class ability that can be used at level 20 or below, and unlimited funding. They can cast on either side of the door beforehand, but once the invasion arrives, they can only cast on their side of the door. They will not engage in open warfare.

How invulnerable can they make this gate door?

The siege has no particular time window or limit, so only effects with permanent duration or that can be refreshed from inside are worthy of consideration. The inhabitants of the stronghold are self-sufficient, and cannot be starved out.

Is it possible to hold this door forever against invaders with similar resources? If not, how long could the stronghold hold out? What level of invaders could be indefinitely repelled?

This answer suggests the hardness of an object can be permanently increased to an absurd degree if one is willing to sacrifice a lot of living creatures to it, using repeated castings of Death Knell and Hardening. It's a pretty evil thing to do, but maybe the door itself was built and hardened by some evil architects and plundered from its workshop by the forces of good. Notably though,

The hardening spell does not in any way affect resistance to other forms of transformation.

So I imagine there are plenty of ways to get around a door regardless of hardness. I'm looking for reliable non-time-limited countermeasures for such methods.

(Also, keep in mind that if the army of darkness should eventually decide to give up and goes home, the gate must be able to return to functional use.)

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  • \$\begingroup\$ So, here’s a question. Can magic be cast through the portal to the demiplane? Assuming that there was nothing in the way. \$\endgroup\$ – Obie 2.0 Nov 11 '17 at 18:56
  • \$\begingroup\$ @Obie2.0 I'm not sure if there's an official rule one way or another, but my question is based on the assumption that each party can cast on the side of the door facing them. \$\endgroup\$ – Makst Nov 11 '17 at 21:34
  • \$\begingroup\$ Is the original creator of the demiplane available, to make more planar modifications? \$\endgroup\$ – topquark Nov 12 '17 at 2:45
  • \$\begingroup\$ @topquark Sure, I don't see why not. \$\endgroup\$ – Makst Nov 12 '17 at 3:04
  • \$\begingroup\$ Remember this maxim "If it has stats we can kill it." This is the most important factor in these sorts of questions, eventually a group of PCs can trash anything with a stat block, ditto sufficiently motivated and well-built NPCs. \$\endgroup\$ – user40081 Nov 15 '17 at 12:20
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I submit that there is no way to guarantee that an arbitrary number of 20th level PCs with an arbitrary amount of wealth cannot do X, for any given value of X, short of direct divine intervention (read: GM fiat).

For the attackers, X is either "gain entry to a demiplane created via Create Deimplane" or "bypass a door". For them, magics like Mage's Disjunction, Wish, and Miracle are extremely powerful: any one of them will eventually remove any magical protection on the door and the latter two will also eventually be able to grant entry to the demiplane as by Plane Shift/Gate.

On the other hand, the denizens of the demiplane are perfectly safe: for them, X is "repel entry by the bad guys". Again, Wish and Miracle will be useful, as might Wall of Force (helps keep out pesky ethereal creatures), Teleport Trap, Dimensional Lock, and even Antimagic Field and Contingency. The defenders make an arbitrary number of similarly-impassable doors, and use Contingency to add an effect to them that causes the next one in line to be put in place when the current one is destroyed. Or, instead of a door proper, put the gate in a Golem of some sort (Adamantine and Quintessence look interesting) and tack on some extra protections (probably including an item that spams Make Whole on said golem).

The best you can hope for, mechanically, is a Gambit Pileup (warning: tvtropes) as the various measures, counter-measures, counter-counter-measures, etc. all vie for control, or the amazing cleric fight from OOTS. That is, both sides have their own Pun-Pun, both of whom fight it out for all of eternity.

At the end of the day, though, the GM will have to decide whether the sanctum is breached, and the decision will almost certainly be based on which outcome best serves the story.

Pathfinder (along with probably all other RPGs) breaks hard when the limits are removed. I've played this game whilst imagining how to protect a 3.5 demilich's phylactery; eventually, everything included "and a way to summon the demilich back if all else fails".

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  • \$\begingroup\$ Bummer. I really would have liked to have an "ultimate siege" as part of my campaign world without having to resort to dm fiat. \$\endgroup\$ – Makst Nov 13 '17 at 20:01
  • \$\begingroup\$ @Makst: there's always GM fiat. Instead of fiat-ing which side wins, though, place limits on what the other side can do. 20th level NPCs are typically rare in a campaign world; maybe the door just needs to hold off thousands of lvl 1-5s with the occasional lvl 10-15. Victory could be the PCs sneaking out to repair the door or destabilize the big bad. Or, put a timer on it: the big bad has a McGuffin that will bypass the protections on the next full moon; destroy the McGuffin and/or big bad by then or all is lost. "How do I make a siege interesting" might be an excellent question! \$\endgroup\$ – minnmass Nov 14 '17 at 14:09

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