The Introduction of magic into a D&D Economy

[Jack_Simth]

Sure. But most people won't willingly live in what amounts to the wild west without any real hope of something profitable coming out of it.

An awful lot of people left paying jobs to move out west and farm. You don't necessarily have to have a captial "c" City to be able to fend off most of the beasties that will think the locals are a prize worth eating (and a Tippy-city will still have some problems if a few Spectre's walk under the city streets to locate a nice large batch of commoners to eat in relative privacy, even if it's not a completely fatal happenstance). A reasonable wall, cleared land sufficient to give a lot of warning of incoming critters, a system of bells and watchmen, and having everyone go around armed with both ranged and melee weapons will take care of most beasties, unless they get attacked in-mass (which is always a problem anyway). Exactly how disparate the village is from the City in terms of survivability will depend on just how bad things are outside... which isn't well-defined in RAW, and is quite campaign dependent.

True. But you can have your forces meet the enemy forces at sea.

Only if you can see them coming. Have fun watching under the water, or during a storm. For every defensive measure, there is an offensive countermeasure. They just become progressively more expensive. And generally, defense is more expensive to begin with.

The guy who rents a room and digs down to establish a beachhead? You can potentially locate him in advance by way of abusing Contact Other Planes to get info about anyone planning to do something like that. Even if you don't catch him in advance, though, there's a limit to how fast they can move various sorts of soldiers through that beach-head. If you have a way to detect that such an event has occured (Lawful-Neutral, Rebuking Cleric-6's that Commanded Shadows, having them patrol under the city, perhaps), you can then counter with overwhelming force (as you have considerably less limitations on how many of your soldiers you can move - after all, you've prepared for this sort of thing occurring, if not for the specific choice of beachhead).

There's going to be a nasty fight, sure. Lots of casualties, sure. But then, there's also no real way to stop someone nuking a modern city by way of, say, smuggling in a nuke in with a very large shipment of something else, and setting it off in the harbor before (or when) the inspectors get there.

Nuclear weapons have been used as weapons exactly twice in recorded history - Hiroshima and Nagasaki - for the simple reason that nobody wants to face a world where they are used a lot (a variation of trust, if a somewhat twisted version of it). Set things up so that anyone who pulls such an attack may do massive amounts of damage, but won't survive the repercussions, and you can have an amazing amount of peace. Once you've got that set up, all you have to worry about are the people who aren't clever trying it - and they won't generally be the ones thinking about having someone immigrate, rent a room, and dig a pit for a beachhead.

Except that traps and trap prices are RAW allowed. Unlike all the other magic items their prices aren't guidelines. Making an at will wondrous item of Forbiddance requires DM approval and is effectively a houserule. Blocking the True Res trap likewise requires a houserule.

Traps show up in the DMG, not the PHB; it's for the DM's use, first and foremost. All traps are supposed to be approved by the DM, even if it's not explicit. Likewise, the DMG is quite clear the PrC's are supposed to be tightly controlled, but almost everyone pretends like they're normal character building and the players should have easy access to them. You generally don't play strict RAW anyway.

And Forbiddance does have the unfortunate effect of killing most commoners of different alignments who enter without knowing the password. And it stops you from making use of teleportation to respond to your enemies beach heads effectively.

Not really; there's no reason to keep the password secret, so you post the password in common on the wall at each boundary. It's a clerical spell anyway, so the password is something like "All Hail to Pelor". Use the same password each time, and make sure the boundaries are clearly marked. Crossing a boundary becomes comperable to crossing the street at an intersection - either can get people killed, but it'll generally be the stupid people, and even at that, it doesn't happen often.

Likewise, you can build your own beachheads to deal with the reciprocal non-teleport issue. There's several stations spaced every so often, where you've got what amounts to a plug in a hole in the ground, with something set to push the plug out of the hole when a particular event occurs (a Telekinesis trap, triggered by a particular pattern of a Dancing Light spell in the sky). Put a bunch of fake plugs every here and there, make sure your teleport response team is familiar with where the real ones are, and then as soon as a hostile beachhead is discovered, you can drop your troops on them.