Nobody knows the origins of the Wall of Thorns, or just how long the Wall has existed. Its existence is noted in various religious and historical tomes that date back over a millennium. Many speculate that the Wall is the creation of the gods; other surmise that it was somehow created by a great empire whose borders spanned where the Wall now resides.
The span of the Wall runs several hundred miles, generally in a north-south direction, separating the Known Lands from the unknown western part of the continent. The Wall ends at a mountain range to the north, and a canyon to the south. No river or stream crosses the Wall.
The height of the Wall varies, but generally ranges from 30 to 40 feet high. It's width, when it can be measured, is usually 10 to 15 feet deep. The branches that make up the Wall are extremely tough and durable, and the thorns themselves are razor-sharp. Moreso, the thorns are also poisonous.
While not a simple thing, the dedicated can usually find a way to make their way to the other side of the Wall, with a bit of effort. The branches and vines can be physically cut and cleared, although this must be done with great care due to the poisonous thorns. They can also be burned, but this is also dangerous, as the smoke given off from such a burning is also deadly to breath. Those with access to magic or sufficient architectural/engineering skill can of course make their way over the Wall.
Whenever a part of the Wall is cut/damaged/burned, for whatever reason, it regrows at a rapid rate, and usually within days is once again whole.
Poisoners and other assassins sometimes journey to the Wall to gain access to the poison secreted from the Wall's thorns. This is not without its dangers - the branches that can remain immobile in hurricane winds can also suddenly sway here and there in the slightest breeze, and more than one would-be assassin has been slain by the very poison they sought to acquire.