The sentry stone is our "artillery entry." We got it the same time that Legion got the Scather, Trolls got the Thumper, and Skorne got the Catapult....
They're costly, points-wise, and they don't have particularly exceptional stats. I've never played them, but they don't pass the eye-test on paper. ...
1 - Great tournament support. Steamroller is a really good system that receives yearly updates to patch oddities that don't work as well as intended...