I picture a tiered garden in three sections. The tall, back section which will be the backdrop for the rest of the garden flowers; the front area which will hold the ground cover plants and low growing annuals and / or perennials; and the middle section which holds everything else!
It would be a bit more organized that a cottage garden where things of all sizes grow amongst each other.
Filling in the central part of the garden may be the easiest task since finding perennials that are average in height may be the easiest. Not only that, but mixing in a few taller varieties here is also okay.
The middle area is a good place to add medium size hydrangeas or some day lilies.
Depending on how large your garden will be, or is, adding a central focal point such as a compact tree or larger bush would be a good idea. I’ve never used tall, ornamental grass, but it can add character to the garden too.
Choose some type of garden ornament or giant pot to be the garden focal point or to add interest.
The middle section could have some peonies (another favorite of mine) and stand alone asiatic lilies to add varying colors. If you love roses, they could be planted in the middle section.
Just keep in mind the different needs of plants when you mix together a tiered garden, or any garden. Roses need loads of sun, and good air circulation around their leaves. They also need to be fertilized regularly.
When planting your tiered garden be sure to give everything room to grow. It may look a little sparse the first year or two, but it can be filled in with annuals until the perennials grow up.