You could skip this step, but I tested the towers by setting up the circuit on breadboard. Wire the anodes and cathodes up in the easiest way. Its not necessary wire them up exactly as per the PCB version for the purposes of testing. I also uploaded the sketch to the ATtiny at this point (although that could be done on the final PCB).
To upload the sketch to the ATtiny, I first loaded ArduinoISP sketch to a Nano 3, then added the "anti-master-reset" capacitor and wired up the Slave-Reset, MISO, MOSI and SCLK pins as described in the ArduinoISP sketch. Procedure should be exactly the same for Uno and most other Arduinos. During programming of the ATtiny, the led cube will blink randomly, because those same pins are connected to the MAX7219 IC.
