Castle is based around 3-bit binary digital signals:

In mathematics and digital electronics, a binary number is a number expressed in the base-2 numeral system or binary numeral system, which uses only two symbols: typically "0" (zero) and "1" (one).
The base-2 numeral system is a positional notation with a radix of 2. Each digit is referred to as a bit. Because of its straightforward implementation in digital electronic circuitry using logic gates, the binary system is used by almost all modern computers and computer-based devices.
The modern...

Imagine 3-bit binary like an 8 layer cake. All combinations of 3 bits create 8 possible values. Ever seen NTSC color bars? If you send 3-bit binary into RGB, you get those colors.
All the inputs on the modules are comparators, so they can perform a 1-bit conversion of an analog signal:

In electronics, a comparator is a device that compares two voltages or currents and outputs a digital signal indicating which is larger. It has two analog input terminals
V
+
{\displaystyle V_{+}\,}
and
V
−
{\displaystyle V_{-}\,}
and one binary digital output
...

The ADC module slices analog voltages up into 3-bit binary numbers:

In electronics, an analog-to-digital converter (ADC, A/D, or A-to-D) is a system that converts an analog signal, such as a sound picked up by a microphone or light entering a digital camera, into a digital signal. An ADC may also provide an isolated measurement such as an electronic device that converts an input analog voltage or current to a digital number representing the magnitude of the voltage or current. Typically the digital output is a two's complement binary number that is proportional t...

If you patch a triangle waveform into the ADC, you get a step pyramid waveform out!
The DAC module converts 3-bit binary numbers into analog voltages:

In electronics, a digital-to-analog converter (DAC, D/A, D2A, or D-to-A) is a system that converts a digital signal into an analog signal. An analog-to-digital converter (ADC) performs the reverse function.
There are several DAC architectures; the suitability of a DAC for a particular application is determined by figures of merit including: resolution, maximum sampling frequency and others. Digital-to-analog conversion can degrade a signal, so a DAC should be specified that has insignificant er...

There’s also a voltage controlled oscillator, these make horizontal or vertical bars in different sizes:

An electronic oscillator is an electronic circuit that produces a periodic, oscillating electronic signal, often a sine wave or a square wave. Oscillators convert direct current (DC) from a power supply to an alternating current (AC) signal. They are widely used in many electronic devices. Common examples of signals generated by oscillators include signals broadcast by radio and television transmitters, clock signals that regulate computers and quartz clocks, and the sounds produced by e Oscil...

You combine the binary outputs to make complex patterns with these:

In electronics, a logic gate is an idealized or physical device implementing a Boolean function; that is, it performs a logical operation on one or more binary inputs and produces a single binary output. Depending on the context, the term may refer to an ideal logic gate, one that has for instance zero rise time and unlimited fan-out, or it may refer to a non-ideal physical device (see Ideal and real op-amps for comparison).
Logic gates are primarily implemented using diodes or transistors actin...

It gets freakin crazy when you start messing with the counter, shift registers, and flip flops, which allow you to program memory states and reveal the true digital nature of everything:

In electronics, a flip-flop or latch is a circuit that has two stable states and can be used to store state information. A flip-flop is a bistable multivibrator. The circuit can be made to change state by signals applied to one or more control inputs and will have one or two outputs. It is the basic storage element in sequential logic. Flip-flops and latches are fundamental building blocks of digital electronics systems used in computers, communications, and many other types of systems.
Flip-flo...

In digital logic and computing, a counter is a device which stores (and sometimes displays) the number of times a particular event or process has occurred, often in relationship to a clock signal. The most common type is a sequential digital logic circuit with an input line called the clock and multiple output lines. The values on the output lines represent a number in the binary or BCD number system. Each pulse applied to the clock input increments or decrements the number in the counter.
A...

It follows LZX standards, for patching and interfacing video signals, and requires Cadet 1+Cadet 2 or Visual Cortex as core modules:

[WORK IN PROGRESS, FEEDBACK IN THREAD WELCOMED]
LZX Industries Patchable Video Standard V1.0
The Patchable Video Standard proposes an electrical and interface specification for wide bandwidth analog computing instruments intended for creative and expressive applications. It is optimized for, but not limited to: (1) generating and processing analog RGB video graphics in SD resolutions, (2) affordable devices which are accessible to working artists and not just big studios, and (3) maximum patcha…
Read up a bit on video sync and how the raster works:

https://www.maximintegrated.com/en/app-notes/index.mvp/id/1184

Stay inspired and have a creative goal to challenge yourself with:

And learn some history:

PCjr/Tandy Graphics Adapter
Enhanced Graphics Adapter
Multi-Color Graphics Array
The Color Graphics Adapter (CGA), originally also called the Color/Graphics Adapter or IBM Color/Graphics Monitor Adapter, introduced in 1981, was IBM's first graphics card and first color display card for the IBM PC. For this reason, it also became that computer's first color computer display standard.
The standard IBM CGA graphics card was equipped with 16 kilobytes of video memory and could be connected either t...