How can I set up a long-range alarm for my fortress?

Question

My wizard is building a large, magically defended fortress. I have many powerful wards and defenses protecting it, but there's one large weakness - if I'm not in my fortress, I have no way to tell if someone is trying to break in. I'd like a way to be alerted that there are intruders in my fortress while I'm out saving the world. This can consist of a spell, a series of spells, or any other complicated shenanigans involving spells and magic items as needed.

Here are my specific requirements:

• I need the alarm to be activated by the presence of creatures in a specific area. Ideally it would work like Glyph of Warding where you can set specific conditions and exceptions, but I'd be willing to accept an alarm that simply triggers if anything enters the area.
• The alarm needs to be able to alert me over long distances. Ideally, it would at least reach anywhere on the same plane. If such a solution doesn't exist, an alarm that I can detect over distances measured in hundreds of miles would work.
• The alarm needs to be long-lasting. Ideally it would be permanent, but at bare minimum it needs to last for a month before I need to return to my fortress.
• The alarm signal needs to reach me relatively quickly. I need to have time to get the message, teleport back to my fortress, and confront the intruders. A signal or message that takes days to reach me will not meet this requirement.

Here are my constraints and resources:

• Anything in DnD Beyond that is not adventure-specific is legal - no homebrew or UA
• Assume the spellcasting ability of a level 20 Wizard with any spells on Wizard list available
• Assume I can obtain the limited services of a level 20 spellcaster from another class (i.e. I could call a favor to have a Cleric set something up once, but I can't have them come by and refresh the spell every week)
• Monetary cost and material components are inconsequential, although lower cost is obviously preferred
• Limit to six months of prep and setup work
• Use of magic items from the DMG as part of the alarm system is permitted, but limited to one item that requires attunement
• Assume I have the resources and companionship of several other fourth-tier characters (level 17-20) to help if I need to undertake a particularly dangerous quest for a component of the alarm system

If no answer meets all of these requirements and constraints, please come as close as you can. I've spent some time trying to puzzle out a way to make this work and I'm stuck - it's easy with a fairly reasonable homebrew, but within the constraints of spells and items we already have, I can't see a way to make it work.

Answer 1

Leave a Simulacrum of yourself guarding the fortress

1. Cast a Simulacrum of yourself
2. Provide it with a copy of your spellbook
3. Have the Simulacrum keep ritual casting Alarm in the relevant areas.
4. The Simulacrum can cast a Sending to you when the Alarm is triggered.

This has a setup time of 12 hours plus however long it takes to copy the spells you want to provide your simulacrum and needs to be renewed when the Simulacrum runs out of slots to cast Sending.

Alternatively you can provide your Simulacrum with a more easily renewable Sending resource:

1. You can give your Simulacrum a ring of spell storing which you can renew yourself.
2. Provide the simulacrum half a Sending Stone set (as suggested by Slagmoth)

Other considerations

Your Simulacrum can keep casting other rituals spells you provide it.

This expends your single simultaneous Simulacrum limit unless you use a method such as described here.

Answer 2

Hire guards who can cast Sending.

Sending works for any distance, even across planes. It has a 5% chance of failing across planes, but since you get to answer, the spell failing can be noticed and compensated by repeated castings.
Hire multiple guards and pay them well to reduce risk of defecting or them getting taken out before being able to send a message. If you cannot find suitable magic users willing to waste their life in your fortress maybe compensate them by giving them limited access to your library. Alternatively acquire some Item of Sending so you can just hire commoners.
As far as I can tell it meets all your requirements and is very cheap on resources compared to what you are willing to pay.

Answer 3

Depending on the climate around your stronghold, another very inexpensive possibility would entail:

1. A nice forbidding swamp surrounding your keep

2. Lots and lots of frogs living in said swamp

3. A faithful Bullywug manservant with good hearing

4. A pair of Sending Stones, as per @Slagmoth's suggestion in a comment

The Bullywug can naturally speak with Frogs and Toads, allowing for an all-natural alarm system surrounding your fortress (can't be dispelled, and unlikely to even be noticed by an intruder). Your bullywug then only needs to croak at you through the sending stones when anything unusual wanders into the swamp and gets the frogs gossiping.

Answer 4

Use guards and wards in conjunction with a sending stone.

Place a suggestion that who ever enters (who is not you) pick up the stone of sending, activate, and declare "Visitors have arrived." The other part of the suggestion is that they then return the sensing stone to its proper space

As guards and wards can be made permanent, and sending some when's so long as the wizard us in the same plane, this should work.

Answer 5

Put Scrying or Sending into a Glyph of Warding

Glyph of Warding states

The spell being stored has no immediate effect when cast in this way. When the glyph is triggered, the stored spell is cast. If the spell has a target, it targets the creature that triggered the glyph.

Scrying

Scrying targets the creature you scry on. You need to upcast the glyph to 5th level, as it is a 5th level spell. There are also some downsides, like it potentially disrupting concentration on another spell you may be concentrating on at the time, or the creature making its saving throw. In exchange, if the target fails its save, not only do you get notified, you also directly see the intruder.

(Unfortunately, I do not think you can use the "location" mode of scrying, as a location is not the same as an area. If your DM allows to use them interchangeably, you can use that and get rid of the saving throw. The area you see will be centered on the intruder).

Sending

As an alternative that does not have a saving throw and also does not break concentration, you can put sending into the glyph. It likewise will target the intruder and let you send a message to them.

There are two mentions of casting the spell with a spell glyph in glyph of warding:

You can store a prepared spell of 3rd level or lower in the glyph by casting it as part of creating the glyph. The spell must target a single creature or an area. The spell being stored has no immediate effect when cast in this way. When the glyph is triggered, the stored spell is cast.

The question is when you need to select the message you send - when you originally cast the spell to store it into the glyph, or when the glyph is triggered? In the latter case, you need to formulate the message when it triggers, and thus you know it was triggered.

Sending says the the storing of the spell has "no immediate effect". The Spellcasting rules on p. 202 PH say:

Each spell description in chapter 11 begins with a block of information, including the spell’s name, level, school of magic, casting time, range, components, and duration. The rest of a spell entry describes the spell’s effect.

The formulation of the message happens as part description section of the spell, so it is part of the effect. As storing the spell has no immediate effect, you select the words when the spell goes off, not when you store it¹. Therefore, you would get notified, and could send a nastygram to the intruder, for example forewarning them of their imminent doom if they do not leave your premises.

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¹ There also is this Q&A in the Sage Advice Compendium, that indicates at least targets for spells are only selected upon completed casting:

If a spellcaster is affected by slow and takes two turns to finish casting a spell, what happens if their target has moved out of range or out of sight? You choose the targets of a spell when you complete casting a spell, not when you start.

This likewise suggests that parameters in the effect description are selected upon completed casting.

Answer 6

You could cast glyph of warding, storing a sending spell with you as the recipient.

Answer 7

How about a spectator?

According to the Monster Manual (p. 30), spectators are...

Magical Guardians. A summoned spectator guards a location or a treasure of its summoner's choice for 101 years, allowing no creature but its summoner to enter the area or access the item, unless the summoner instructed otherwise. If the item is stolen or destroyed before the years have all passed, a summoned spectator vanishes. It otherwise never abandons its post.

This is up to your DM, but I think a castle should qualify as a location and while it is not exactly defined how it guards the place, I assume it doesn't slack nor sleep. It can also be supported by human guards if needed (He would make sure they don't slack).

The means of notifying you are as in the other answers. Since it can speak common, you could provide it with a sending stone mentioned in other questions to reach out to you in case of danger. Alternatively he should be able to cast sending from a ring of spell storing, however that would have the 5% chance of failing over different planes.

In order to summon a spectator you would need...

a magical ritual, the components of which include four beholder eyestalks that are consumed by the ritual's magic

Not part of the Monster Manual, but on the Forgotten Realms fandom wiki they mention that:

Spectators disliked guarding things for lesser lifeforms, which most mortal summoners would qualify as, and would only do so hesitantly. After promising to do so, however, their loyalty to their summoner was absolute, guarding the object in question until their time was up.

Answer 8

Contingency : A cheesy alternative

Cast a Contingency spell with e.g. Blur or Disguise Self as a contingent spell (really any spell that you would notice immediately, but maybe avoid Fireballing yourself), and "whenever an unauthorized person steps into my lair" as a triggering condition. Contingency doesn't have any restrictions on its trigger unlike spells like Seeming :

The triggering condition can be as general or as detailed as you like, though it must be based on visual or audible conditions that occur within 30 feet of the area.

So RAW, as there are no hidden rules, the contingent spell can be triggered at any distance from your actual lair, even on another plane.