it actually works well with axoloti.. the Kastle on its own I found a bit noisy but once fed into axoloti, the noise seems to go. (I think its due to the really high gain levels on the Kastle)
Not tried physically connecting the Kastle yet, should be ok to connect GPIO directly to the input, but you'll only get 60% range, as I think its 5v vs axo 3.3v - similarly you'll need a voltage divider to scale the output connector to send it to axo.
Its on my list of things to do
yeah its alot of fun... and at the same time, I got an OMSynth, so will also be bringing this into the mix