I know that in the Hobbit spider webs did not cross the Elf path, so I assume there was some enchantment there, but what was the enchantment and how powerful?

Any thoughts?

HP

I believe it has something to do with the natural enchantments of Elf created items and the way they react with creatures of the Shadow. As to the power level of the enchantment the Elf Path in our campaign was enough to stop the smaller giant spiders in their tracks, but did little to stop the Werewolf of Mirkwood from crossing it; at least from the tracks we found anyway.

The simple answer is: we don't know.

From a mythic / fairy-tale perspective, it's the story value of the direction "dont leave the path!" that's so strongly instilled in our ancestral psyches that's the most important. That would tie in neatly with the sense of the living, mythic world of Middle-earth that Steve Long so elegantly captured for Decipher's LOTR RPG as "the subtle magic of Middle-earth". And hence the Elves themselves might not even be able to say 'how it works' except that they do their thing -- singing, presumably -- and Middle-earth responds with the phenomenon of the path being safe in any of a number of ways.

Alternately, and a bit more objectively, I like to think that 'Path Magic' involves tending and nurturing the natural world to dispel the Morgoth-element from the earth and the plant-life in the immediate vicinity of the path, rendering it as close as their power allows to how Eru and the Valar intended it, before the influence of Melkor resulted in the degradation or contamination that is 'Arda Marred'. Taken to its extreme, and wrought with sufficient power -- and possibly a little something extra -- I think this is what "Hallowing" is.
The creatures of Shadow find this (near) perfection distasteful and simply tend, knowingly or unknowingly to avoid it. Compare the statement made of Rivendell, that "bad things do not come into that valley", or the description of the fully Hallowed Amon Anwar in UT's Cirion and Eorl chapter. I would interpret it on roughly the same sort of lines as the aversion of Orcs and Goblins to the light of the unsullied Sun; they don't like it and tend typically to avoid or shy away from it, but if motivated to make a bit of an effort of will -- spend a Hate point? -- they can defy the warding effect and act however they like, but perhaps at a penalty for that discomfort.

Of course the Elf-path through Mirkwood is far from a woodland idyll; not all aspects of the natural world of Middle-earth are intended to welcome and delight the Children of Iluvatar. The Elves will appreciate such manifestions of nature in their own right, but not even lore-masters amongst Men and Dwarves are likely to find them anything other than discomforting.

Some gamers might prefer the completely objective interpretation: that the Elves cast permanent 'Ward Spiders' and other spells over the path (or perform periodic maintenance of near-permanent ones). But I think that would have been far too mechanistic for Tolkien's tastes.

Cheers,
--Os.

Don't forget that the Elves use the path themselves and most likely keep it safe. Elven arrows are just as good a deterrent as enchantment for that purpose.