So this is all with the original kastle, not the AE version. (I do have the AE one, technically, but it's arrived well ahead of my AE case...) Anyway. For patches, try:
* Start by taking the osc out or secondary osc out, and run it straight out of you system. Now you can hear what you're doing.
* The classic kaestle patch starts with:
Bit out -> pitch in. This gives a kind of sequence. Playing with the pitch in attenuator lets you sort-of tune it. You can then try either LFO patch point, or another bit out, to modulate timbre/waveshape over time.
* LFO square out -> LFO reset in. This gives a ramp down wave at the LFO tri out, that's good for pretending to have envelopes. (Remember that the original is a self-contained system: it can do neat things all by itself, but some of them are going to be less useful in the context of a bigger modular.)
* LFO square out -> LFO tempo in. This gives swing.
* You can also do more 'arp-like' things with LFO square out to pitch in. It's maybe a touch boring by itself, but this frees up the bit out to use elsewhere while still getting rhythmic pitch changes.
* There's a Krell-ish style of patch available with the bit out to the tempo in. Dial the tempo attenuator up a bit until the LFO is giving a good range of short and long pulses (and also patch the LFO into the osc somewhere so you can hear it).
* Patching constant sources into the mode in lets you hear/use the other modes. Patching your sequencer pitch (eg. the bit out) into the mode in can give you mode-per-step pointillism/'accent'. Patching the oscillator outputs into the mode in gives a noisy lo-fi mess that has chaotic-attractor 'stable' spots if you play around with the timbre and waveshape.
* You always have two osc modes sounding at once: try mixing them. The track-and-hold can make a nice bass tone under the chord-like formant mode.
* Probably you can do osc out -> tempo in, then use the bit out as a wavetablish osc.
Also I highly suspect that, for the default PM/PD modes especially, it'll benefit a *lot* from having envelopes on the timbre and waveshape (and a filter on the output). You might be able to get more "traditional" FM-ish sounds that way.
The thing to remember is that it's noisy, lo-fi, intended to self patch, untunable, and chaotic. It's hard to tame. It benefits from patching in loops -- try audio out into all the audio parameters including mode, all the LFO/stepped outs into the LFO tempo/reset, etc. Also it's sweet spots are tiny and fiddly, it'll sound great with the knobs in very exact spots then the slightest tweak will mess it up.
... that's all from memory, I don't have my kastle on me (or case/power for my ae one). Some of these tricks are in the kastle manual. I'll write up patch notes for some specific favourite ones when I'm back home, maybe.