Can I use modeling chocolate over rice krispy treats to make turrets for a castle? Would it be possible to use an impression mat to get a brick or cobblestone effect?

yes to all that you asked. Modeling chocolate can be rolled out like fondant to cover a cake.

About chocolate and impression mats, there's quite a bit of information and says you can use the impression mats with chocolate at http://w w w.su gar cra ft.c o m/ca talog/gum pas te/impr ession-m ats/impr ession-m ats.h t m
(I've added spaces in hopes that CC won't block the address.)

At Joy of Baking, I read this:

Modeling Chocolate
This is a pliable chocolate paste made from just two ingredients, chocolate and corn syrup. It has the texture of a tootsie roll or marzipan and is very easy to work with. It can be used to make ropes, braids, ribbons, ruffles, flowers, or leaves. Can be made with bittersweet, semi-sweet, milk or white chocolate.

I personally don't have any experience with either impressing or modeling wtih chocolate.

HTH, Shanter

I've been doing all my figures in modeling chocolate. It taste better than fondant.