Place the longest resting shelf where it offers comfortable access, enough surrounding clearance and a position your cat is likely to enjoy. It does not automatically need to be the highest point.
A broad bed at a moderate height may be more useful than one close to the ceiling with restricted headroom. Check how the cat enters and turns on the shelf, and whether resting there would block the only route for another cat. The wall support and the product's installation requirements determine which locations are actually available. Consider sunlight, household activity and nearby windows without making the bed an unsafe route to an opening. Keep maintenance access practical too. Choose the position after drawing the complete route, rather than installing the largest shelf first and trying to fit all the connecting steps around it.
Mark a reclining cat's outline on the proposed shelf diagram and check both approach directions. A spacious bed can still be inconvenient if its entrance sits behind a support or if every other cat must cross it to pass.
For product comparisons alongside this advice, see SkyRoute wall climbing set and Feline Sovereign Spire. The details below explain their sizes and uses.
Further reading: vcahospitals.com; cats.org.uk.
SkyRoute wall climbing set
Feline Sovereign Spire