Play can help a cat discover lower platforms if it stays gentle, voluntary and suited to the cat's movement. Begin on the floor beside the tree, then let the cat notice an accessible surface without making a sudden or difficult jump. Keep any toy movement within a stable landing area and stop if the cat hesitates or loses interest. Do not dangle a toy over an exposed edge or encourage rapid leaps between levels simply to show that the tree can be climbed.
Put string-like interactive toys away after the session rather than leaving them attached unattended. You can also let the cat investigate quietly between games; exploration does not have to become a performance. If it repeatedly avoids the same step, check the spacing, surface and stability. Play is an introduction method, not a way to override an uncomfortable layout or a possible mobility problem.
Begin with the toy on the floor beside the tower and finish the session there too. Let the cat choose any low step between those points; the purpose is a pleasant association with the area, not reaching a particular height.
For product comparisons alongside this advice, see Ultimate PawHouse and Taj-MeowHall. The details below explain their sizes and uses.
Further reading: cats.org.uk; rspca.org.uk; cats.org.uk.
Ultimate PawHouse
Taj-MeowHall
Cat-Cave