Thanks for your replies!!

My wall is about 3' high & 25' long- hasn't moved a hair in 5 NY
winters. * It is serpentine & I think that adds to both appeal and
strength.

Jim, that's good to know! I like the idea of serpentine walls too. I'm
concerned, though, that building one wall above another will create
additional issues over and above the construction of one wall--what do
you think?

Norminn, the slope is away from the house and will stay that way. we
have a flood plain easement on our land and in fact the neighbors'
gardens drain into that so, I don't think it will be a problem. Also
we're thinking of putting in a pipe from the lowest terrace to
encourage water down to that area. What sort of plants did you use to
control erosion? And how quickly do they establish a root system
sufficient to have some effect?