Swordsage, how to go about it

[xigbar]

Although I would rule in your favor, I don't think they technically create difficult terrain under RAW.

The effects of Caltrops in and of themselves are similar to hindered terrain. Adding in marbles makes it even more so.

 

[kitcik]

The effects of Caltrops in and of themselves are similar to hindered terrain. Adding in marbles makes it even more so.

True but, under RAW, Step of the Wind wouldn't help you so you would not have achieved the desired result.

Step of the Wind
"... incurred by moving through difficult terrain."