How would you handle caltrops in your game?
Carefully - they're very sharp.
I was thinking about treating them as a trap. Upon entering an affected area (and for each round spent in that area) the PC's risk stepping on them on a 1-2 of a D6. Upon suffering damage (a d4 - I realize that spikes are a higher damage step, but I thought the spikes on caltrops would be smaller), PC's are subjected to 1/2 movement rate until healed. Is that fair?
I like the movement penalty. For smaller caltrops, I would think the damage far too steep but there would be some penalties to spot them. For any damage effect, I try to visualize the worst case; death would be extreme for caltrops, but a d4 could easily off many low-level adventurers (especially if already wounded).
nothing wrong with a d3, d2, or possible 1d4-1 for the possibility of not being damaged.
This came up in my game earlier for a thief who wanted to use some, and I also decided that a caltrops were basically a trap and were readily available to thieves' guild members. A creature without heavy foot protection moving through an area containing caltrops at combat speed or faster had a 2/6 chance to step on a caltrop, slowing them to half-speed until the wound is treated (which takes at least a turn). I didn't have my caltrops do damage, but a suitably devious wielder might poison them (treat as ammunition), and I bet rusty caltrops give you tetanus about 5% of the time! If a creature is moving slower than combat speed, the caltrops are obvious and avoidable.
I priced a bag sufficient to cover a 10-foot square as 2gp and one stone. There are anti-cavalry variants at use in the Empire's legions, but I haven't statted those out yet. :)