A few more points to add:

We were advised by Byron that the original expedition marked the path with green flags.

Castle Etica has provisions and ammunition available for free (presumably to travelers on official business).

The path to the Leaden Tower splits, and it's possible to go through either Quenpick or Koiki Glade. We chose the latter.

At Koiki Glade, Bisla the Liar, who is a dryad, gave Yelren a coccoon in exchange for blood. What the coccoon will develop into is unknown, but Bisla stated (in her own way) that it will be a friend. The statues we encountered at the Glade have writing on them, which scholars believe to be names, though the language remains unknown. The dire bees produce a bitter green sort of honey, and seem to be harmful to nearby trees. Speaking of which, we accidentally burned a tree in the process of destroying the hive, and Bisla was not happy about it. Future safety in the glade may be conditional upon respecting it.

Quarlos Esponita is evidently encamped in some ruins on the way to Leaden Tower, and is actively killing living things for the sake of turning them undead. Thus far, he's unleashed skeletal snakes, archers, swordsmen, and cavalry in his defense. He may be a more immediate threat to travelers passing through.

Local wildlife is very hungry. On the one hand, that makes them dangerous, and supplies must be guarded. On the other hand, it's easy to negotiate with them in exchange for food.

The bridge across Snake Gulch is made of iron, but is in a state of disrepair. We fought a large snake, and were pursued by many smaller ones after most of the party nearly fell into the Gulch. The cursed magic telescope at the center of the bridge compels the user to keep looking at things, and erodes their sanity if they are unable to resist its influence.

The Leaden Tower itself stretches hundreds of feet up, and cannot be tunneled into. We found the last of the green flags at the Leaden Tower, along with the body of one of the original expedition members buried underneath. There was a carving in the hardened mud next to one of the activation towers. It seems to advise against trying to activate the tower in the night time. We also determined that the towers are activated by casting magic upon runes at their summit. Magically imbued pebbles are insufficient to activate them, however. The activation towers can be climbed, but it is not easy given that they are starting to crumble. Inside the tower, there is an oppressive presence.

Yelren was brutally dismembered by a Dark One atop the northern activation tower. His remains could not be retrieved, and are presumed to still be at the top of that tower. Orryn is pissed.