Wish there were a bit more instructions on how to get it running using lein2 / project.clj. Maybe that happens this weekend :-). I am a bit at a loss how to start the webserver used by c2 (and if its even included) from within emacs/slime/swank.
To use within Clojure, just add c2 to your deps as usual and then
(ns your-ns
(:use [c2.core :only [unify!]]))
https://github.com/lynaghk/cassowary-cljs-demo
for a rough demo.