It's only like Turret Defense in the sense that they are both UMS maps. Bound maps are nothing like Turret Defense as far as gameplay goes.

http://youtube.com/watch?v=euEqo2nRz4M

There's an example of a bound. I don't know that specific one and didn't watch much of it, so it might be a bad example. Either way, that's what a bound essentially is.