Does forbiddance have a distance limitation?

I’m a smuggler, and have just completed digging a mile and a half long tunnel from the shore to the city. I don’t know much about magic, but I do know that I don’t want the recent zombie problem to be able to gain access to my tunnel. I hire a Cleric to keep them out permanently, and they choose to cast forbiddance on the tunnel.

Forbiddance states (PHB p.243):

You create a ward against magical travel that protects up to 40,000 square feet of floor space to a height of 30 feet above the floor.

The tunnel is 10 feet high, 5 feet wide, and 1.5 miles long, or 7920 feet. This makes its total area 39600 feet, so it would be within the size limitations. Is there any restrictions on how far Forbiddance can affect? Assuming the tunnel is completely unlit, the cleric could only see as far as there darkvision; would the lack of vision have any effect?

What do you touch for the Forbiddance spell?

Touch spells typically either specify the target that you touch (i.e. Magic Weapon) or involve a spell attack where the rules of targeting are governed by the chapter on spellcasting:

Some spells can target only a creature (including you) that you touch.

Forbiddance doesn’t specify what you touch or whether you must. How do I cast this spell? What do I target?

Range touch […] You create a ward against magical travel that protects up to 40,000 square feet of floor space to a height of 30 feet above the floor. For the Duration, creatures can’t teleport into the area or use portals…

Can I teleport within an area warded by Forbiddance?

Can you teleport within an area warded by Forbiddance? It’s already been established you can teleport out of it, but not whether, once inside, you can teleport from one part of it to another.

You create a ward against magical travel that protects up to 40,000 square feet of floor space to a height of 30 feet above the floor. For the duration, creatures can’t teleport into the area or use portals, such as those created by the gate spell, to enter the area. The spell proofs the area against planar travel, and therefore prevents creatures from accessing the area by way of the Astral Plane, Ethereal Plane, Feywild, Shadowfell, or the plane shift spell.

The phrasing uses both phrases, "into" and "enter". If you teleport from one area to another, you are going into an area, and if you enter an area that would only block teleportation from outside.

As such, I’m not sure whether the spell could be used, say, to ward a battleground so that the enemy couldn’t teleport around within said area easily, or used within a mage’s private area to prevent an enemy teleporting around smashing things.

So, can I teleport within an area warded by Forbiddance?

Can Passwall be used to enter an area protected by Forbiddance?

Let’s say the BBEG has their Tower of Doom protected by a Forbiddance spell and rather than kick down the front door, the party wants to sneak in all ninja-like. The relevant wording in the Forbiddance spell description says:

You create a ward against magical travel…For the duration, creatures can’t teleport into the area or use portals…to enter the area.

I’m assuming Dimension Door wouldn’t work as the description starts off with

You teleport yourself…

But, what about Passwall?

A passage appears at a point of your choice that you can see on a…surface within range…

Does the magical travel wording of the Forbiddance spell prevent Passwall from working, or can Passwall be used to enter an area protected by Forbiddance?

If I attempt to Teleport into an area warded by Forbiddance, is my spell slot expended?

From the Forbiddance spell description:

For the Duration, creatures can’t tele⁠port into the area

A character may not know the area is warded by Forbiddance, attempt to cast Teleport to enter it, and fail. If they do, does that cost a spell slot?

When a character casts a spell, he or she expends a slot of that spell’s level or higher

So my question is: if creatures can’t teleport into the area, can they even cast Teleport targeting the area?

As a DM, if a player says “I cast Teleport to enter the area”, do I say “you can’t cast teleport to enter that area”, or do I say “you cast Teleport, but nothing happens”?

If I attempt to Teleport into an area warded by Forbiddance, is my spell slot expended?

From the Forbiddance spell description:

For the Duration, creatures can’t tele⁠port into the area

A character may not know the area is warded by Forbiddance, attempt to cast Teleport to enter it, and fail. If they do, does that cost a spell slot?

When a character casts a spell, he or she expends a slot of that spell’s level or higher

So my question is: if creatures can’t teleport into the area, can they even cast Teleport targeting the area?

As a DM, if a player says “I cast Teleport to enter the area”, do I say “you can’t cast teleport to enter that area”, or do I say “you cast Teleport, but nothing happens”?

If I attempt to Teleport into an area warded by Forbiddance, is my spell slot expended?

From the Forbiddance spell description:

For the Duration, creatures can’t tele⁠port into the area

A character may not know the area is warded by Forbiddance, attempt to cast Teleport to enter it, and fail. If they do, does that cost a spell slot?

When a character casts a spell, he or she expends a slot of that spell’s level or higher

So my question is: if creatures can’t teleport into the area, can they even cast Teleport targeting the area?

As a DM, if a player says “I cast Teleport to enter the area”, do I say “you can’t cast teleport to enter that area”, or do I say “you cast Teleport, but nothing happens”?

Can you cast the Conjure Elemental spell to summon an elemental within the area of effect of the Forbiddance spell?

The forbiddance spell description states:

You create a ward against magical travel that protects up to 40,000 square feet of floor space to a height of 30 feet above the floor. For the duration, creatures can’t teleport into the area or use portals […] to enter the area. The spell also proofs the area against planar travel, and therefore prevents creatures from accessing the area by way of the Astral Plane, Ethereal Plane, Feywild, Shadowfell, or the plane shift spell.

In addition, the spell damages types of creature that you choose when you cast it. Choose one or more of the following – celestials, elementals, fey, fiends, and undead. When a chosen creature enters the spell’s area or starts its turn there, the creature takes 5d10 radiant or necrotic damage (your choice when you cast this spell).

When you cast this spell, you can designate a password. A creature that speaks the password as it enters takes no damage from the spell.

The spell’s area can’t overlap with another forbiddance spell. If you cast this spell every day for 30 days in the same location, the spell lasts until it is dispelled, and the material components are consumed on the last casting.

Can you cast the conjure elemental spell to summon an elemental within the area of effect of the forbiddance spell?