[rspoerri]

The article simplifies too much to be insightful. Looks like clickbait for hacker news ;-)

The core loop already being discussed in other comments.

The vertical slice is when you implement _a part_ of the game to be completely finished (like a demo). It's not a step that is always done after the horizontal slice, but might be done before, simultaneously or after.

In smaller projects neither of those two prototypes is fully implemented, rather it's the challenge of the developers to agree on a equal idea / concept of the project you are working on. The game design document is the documentation of these prototypes. Most often all these aspects are worked on in parallel. A change in mechanics influencing the visuals, a story element influencing the mechanics, etc. and vice versa.