Eternia Crystal Stand

That rejection message is about the ground, not the crystal. The game wants a wide floor of real blocks on both sides, and it will not accept platforms directly under the stand. Flat has some give: a step up or down of one tile per tile of distance still passes, so you can build a shallow ramp leading away from the stand and climb as high as you want. Steeper steps only survive a couple in a row before the check fails.

Headroom invalidates more arenas than the floor does. A single stray block roughly nine tiles above the surface is enough to fail the check, so clear the ceiling before you start rebuilding anything below.

• Hammered blocks and slopes count as surface, so you can shape the ground freely.
• Platforms above the floor are fine, which is how you build a tiered arena without breaking the check.
• Blocks turned off with actuators read as absent, so leave an actuated doorway in a wall and switch it solid once the crystal is down.
• Closed trap doors count as floor. Open them after placing and the event carries on.

Activating the stand again during the gap after a wave skips the countdown and starts the next one straight away. Worth using once you are comfortable with the tier, since a full run otherwise spends a lot of its time waiting for nothing.