I tried getting in tough with IAG once before, suffice to say it's not as easy as it should be. They definitely need to do a better job on that front. This makes buying from a local dealer more attractive, since if you have a problem you can deal with them. Failing that, you can also try contacting the distributor for Castle products in your area.
I'm not sure about the binding posts on Castle speakers, but many speakers come with a little plastic cover that blocks the area where banana plugs would go. Try to see if your binding posts have this cover, if they do it'll take some effort to pry them off, but then the banana plugs will become accessible.
I'm not sure about the binding posts on Castle speakers, but many speakers come with a little plastic cover that blocks the area where banana plugs would go. Try to see if your binding posts have this cover, if they do it'll take some effort to pry them off, but then the banana plugs will become accessible.