Prototypes should be at the highest pace layer.

The most important thing for a prototype is adaptability; finding the fun.

Ideally prototyping in a team should all happen on the same pace layer.

If there's one prototype being developed on multiple pace layers, you have two bad options:

1) the prototype goes as slowly as the lowest pace layer, making it significantly less likely to find the viable product sketch.

2) the engineer responsible for the lowest pace layer tries to keep in sync with the faster moving pace layer through sheer force of will.

Like spiderman holding the two halves of the ferry together.

Impossibly stressful, if you take a break for a second you rip apart.