I have learned the hard way that window well building is best done before the backfill. This is especially true with the deeper window wells that are often specified today. For my projects I typically try and use the precast window wells but these have limited sizes. In this case you have to use the site built wood wells on precast brackets. These are solid and a large upgrade over the metal corrugated wells you can buy at the hardware store.
The window well is hanging on the wall and the brackets. Digging these out after the backfill would be a horrible job.