Leave clear space where the cat is expected to arrive when stepping or jumping off each part of the setup, especially at the lowest exit to the floor. Consider the direction of travel, not just the vertical space immediately beneath a shelf.
A landing area should not contain sharp-edged ornaments, unstable stools, food bowls or items that people routinely move into the route. Check it with doors open and furniture in normal use. A chair that is usually tucked away can become an obstruction when someone sits down. Follow the product's layout guidance and adjust for your cat's actual movement. There is no single clearance measurement that suits every cat and route. Observe use after installation and keep the area consistent so the cat is not unexpectedly faced with a new obstacle during descent.
Make the landing area visible in your room plan and treat it as part of the furniture footprint. A tower does not occupy only its base if the normal exit route extends into the adjacent walking space.
For product comparisons alongside this advice, see Feline Sovereign Spire and Modern Vine. The details below explain their sizes and uses.
Further reading: cats.org.uk; cats.org.uk; cats.org.uk.
Feline Sovereign Spire
Modern Vine
SkyRoute wall climbing set