1: For hoth you could use circle barriers with no borders (use multiple to your liking if you want) and you could utilize 3-D Terrain/Props and Tinted Barriers.
Layered terrain are good ways to utilize 3-D in Gimkit Creative.
Maybe buttons on the barrier that teleports you to what seems to be the planet but is actually another room.
Resources:
2: For a tower defense game, you could make moving props that activate and deactivate for the enemy troops and sentries for your towers.
If Sentry-Device interactions were still here, it would be a lot easier for TD-Games because you could use zones and teleporters to teleport them.