Spiky low growing shrubs for under window

[Jez]

How about Berberis (Barberry), Deciduous, semi evergreen, evergreen spiny shrub, grown usually for the yellow/orange flowers and fruits, Deciduous varieties are known for the attractive autumnal leaf colours???

[dampflippers]

I don't know what sort of windows you have (wood or UPVC), but have you thought that you may need to get to them to paint or scrub the frames, or for window cleaning?

And be careful when you are weeding etc. A friend of mine got a septic wound when he got stabbed by a berberis.

Is the one you have chosen evergreen? If not it may look pretty untidy in the winter (although the spines will still be very effective!)

[dampflippers]

Have you thought about cacti on the windowsill inside??