Advanced Caltrops (Alt. rules, an artifact, magic caltrops and more)

[Callix]

I like this a lot. Poisoned caltrops are just perfect for getting PCs to look where they put their feet. May I suggest additional rules:
Caltrops and Charging.
A creature that fails to notice caltrops and is struck by one while charging immediately stops, and must make a reflex save (DC = 10 + 1/5ft speed) or fall prone.

Caltrops and Tripping.
If a creature falls prone in an area covered in caltrops, they immediately suffer attacks from 1d4 caltrops, against which they cannot apply their dexterity bonus to AC. The caltrops get a +1 bonus to the attack roll from the creature's velocity. These attacks do not affect the target's speed. Any slippery effect, such as a Grease spell, imposes a -5 penalty to all caltrop attacks in the area.

EDIT: Typo in Tripping: no y in Get.