The keep, though mini, contains a self-sequencer and two voices. The first voice is a simple pulse, from a Hex Schmitt-Trigger, fed through a vactrol (diy) which can be modulated. The second voice is the result of the two FM inputs of an XR2206 which can be switched (quickly, too!) chained, amplitude modulated and tuned between sine and triangle output. So, two voices the second of which can quickly switch character!

The sequencer is the primary pulse wave fed into a 4040 binary ripple counter. And mixed, or tapped at each output. 4 of the higher frequency outputs are mixed via two pots. Which you can add more inputs to. Or tap. Using a simple linear pot means that you can create patterns consisting of multiple 4040 outputs overlapping.

In short a 'nearly' digital cousin to the kastle.