Another Time and Magic scenario. This one for the elves. I'll call this The Great Wall
of Thickets....

Queen Isealie is a 13th level wizard. Using the Plant Growth spell, she creates an area
of thicket that is 16,900 sq. ft. This could create a barrier 10'x1,690' per casting.
She can cast this spell 4/day, creating 6,760 ft. of barrier per day. Over a mile! In a
month, she creates 202,800' of thicket, or 38.4 miles. There are about 500 miles of
border around the Sielwode, meaning she could surround the entire domain in thickets
created by Plant Growth spells in around 13 months.

While this may seem like a long time, I'd suggest that it really isn't terribly long to an
elf, and it's been 2,000 years since they came into conflict with humans. It also assumes
that there are no other mages in the Sielwode of 7th level or greater who might help their
queen, which seems pretty unlikely. (From what I can tell there should be around 100,000
elves in the Sielwode. According the the populations stats I've been using 10% of those
elves are of class and level of whom 20% are wizards or multi-classed wizards, that makes
for around 2,000 elven spellcasters.)

Such a barrier would not, of course, dissuade serious invaders--it would only take ten
minutes or so to get through--but it would serve as a pretty good deterrent to casual
trespassers, and would certainly slow down any units who tried to get through the
barrier. How many units could stand ten rounds of heavy fire from elven archers while
they tried to hack their way through the barrier? Plus, who is to say that over the
centuries the elves would not have made that barrier thicker? Given ten years of serious
effort that barrier could be a 100' thick or more.