While not strictly required, Visual Studio Code will be the official IDE used for software development on the Retro Watch.
The next requirement is the SDK and tool chain for the Nordic NRF5340 processor running the watch.
Here is a good video tutorial on setting up the environment: NRF Connect for Visual Studio Code
FPGA Development:
The Retro Watch is powered by an Efinix T20 FPGA and you while there is a robust video accelerator provided you can modify or write your own as desired.
You will need to register your Retro Watch and install the Efinity IDE linked below:
The FPGA development tools require a license which is included with every Retro Watch purchase. The license serial number is included in the packaging but may also be obtained in the Developer tab in the settings menu of the device.