On Teleportation.

The Blue mages of the world of Dirt are very, very good at teleportation. 
They can do all sorts of nifty tricks with it, ranging from Bags Of
Holding and Dimensional Blades all the way through Interplanar Portals and
teleporting armies.  Plus, of course, in the old days they would never
ever have trouble with muddy roads and traffic jams.  For a long time,
"punctual as a Blue Mage" was even a common phrase in some parts of the
world. 

Unfortunately, although Bags of Holding aren't that much of a problem, it
was quickly discovered that many of the other uses of teleportation were
more sources of trouble than anything else.  Thieves, for example, could
teleport into bank vaults, collect the gold, and teleport out again with
none the wiser.  Idiots could open Interplanar Portals to the Demon Planes
and get themselves and a whole bunch of other people messily disemboweled. 
Duelists with Dimensional Blades caused damage to the space-time continuum
itself whenever their blades collided.  And, worst of all, any snotling
warlord with one mage and a couple of hundred men could plunk them all in
the middle of an enemy city at midnight, elevating his status from Warlord
of a tiny fortress to Mayor of a large city in a single night.  When the
Mayors of the large cities realized this last fact, their response was
immediate.  "Something," they said, "must be done, and it must be done
RIGHT NOW." 

In order to discuss what exactly was done, it is necessary first to
discuss exactly how teleportation works.  Many wizards, if you ask, will
tell you that teleportation works by taking you apart into your component
bits, sending those bits at very high speeds through solid objects, mixing
your bits with other people's bits, and then reassembling those bits at
the destination, hopefully in the right order.  Many wizards will tell you
this, but this is only because they are trying to scare you.  In reality,
most Teleport spells aren't really Teleport spells at all: they're Plane
Shift spells. 

When a wizard does a Plane Shift, he leaves the Plane he's on, tumbles for
a nothingth of a second through the stuff that we would call the Space
Between The Planes except that there isn't any actual space there, and
arrives at a different location on a different plane.  A Teleport is done
the same way, except that the destination plane is the same as the arrival
plane, which requires much less energy because it's so much closer. 

"Wait!", the clever reader might interpose at this point.  "If there's no
space between the planes, then how can one plane be closer than another
one?"  Ask any Blue wizard this, and he will reply loftily that
technically this is true, but it still makes the math a lot easier.  Press
him further and he will assert that most of the effort involved in
teleportation is in specifying where you want to arrive, so it saves a lot
of energy if you don't need to specify what plane you want to arrive in.
This is rather an oversimplification, but it is true in one important
respect: Teleportation does get a whole lot easier if, rather than
specifying "I want to appear 845.25 kilometers to the north, 27.3
kilometers to the west, 10.2 meters up, and facing south", one can merely
specify, "I want to appear somewhere over THERE," and let the spell do
whatever is appropriate. 

This is where the fun bit comes in.  There's a kind of rock - like a
magnet, but not - that emits a field that attracts teleportation spells. 
A good artificer can make a platform out of this rock so that, if anyone
teleports to a spot nearby, they will most likely arrive on the platform
with their head and feet in the correct directions.  Even a crummy
artificer can pile a bunch of these rocks together in such a way as to
attract teleport spells over a larger area.  The thing that the Blue mages
realized, and that made these rocks so important, was that this effect can
be made strong enough to attract ALL teleport spells, not just ones of the
"somewhere-over-there"  variety. 

Thus it is that all of the major cities on Dirt, and most of the larger
villages in fact, have a teleportation-intercepting field around them.
Anyone who tries to teleport into the city finds himself in a small room
in the middle of a large building, with lots of soldiers pointing sharp
things at him.  Travellers can enter the city normally, but armies and
demons and other nasties tend to have a lot more trouble.  In particular,
it has become the fashion to hire artificers, Old Magic wizards, and
sorcerors to put interesting traps around and on the teleport platforms
which can be triggered in case of emergency.  Some of the traps can be
quite imaginative. 

So what was the practical effect of all this?  Thieves can't teleport
within cities at all anymore, so they have to rely on other sorts of magic
to do their dirty work.  Idiots can still create Interplanar Portals, but
after the first few idiots tried it, natural selection worked its magic,
and Interplanar Portals in general kind of went out of style among those
who don't know what they're doing.  Dimensional Blades were never really
in style: At first it seems like it would be cool to own a sword whose
blade is a teleportation field, but they're tricky to make, they can't be
used to parry, they do funny things with other teleportation fields, and
it's really embarrassing when stuff starts coming though the field in the
other direction.  Blue Mages don't have it nearly as good as they used to,
since they can't really teleport anywhere but the City Centers.  They can
do that really easily, though, and many Blue Mages make a nice profit
teleporting travelers to other cities in a hurry.  It's still possible to
teleport armies around, provided you make it a point to stay away from
City Centers; however, attacking cities with teleporting armies is
definitely out. 

And that was all the Mayors of the cities wanted, anyway.  So this is a 
happy ending, for them, at least.