Why?

can we hide structures behind the illisuonal hill

Because falling damage hurts.

this spell would be incredible were it not for the casting time

I don’t see why not, as long as the entire structure is covered by the hill. You can’t change how the structure looks, but adding a hill would change the terrain around the structure, not the structure itself. Of course, your hill may look a bit suspicious depending on the structure. For example, hiding a Mighty Fortress with the spell would probably produce a strangely wide and squarish hill.

I was thinking hiding a small camp with hallucinatory hill but i got what you are talking about.

Thanks for replying

oops i fell off a disguised cliff

Quote from hostileeldritch >>

this spell would be incredible were it not for the casting time

 I mean, I don't think 10 minutes is unreasonable to create a massive 150-ft cube illusion for 24 hours. If anyone were to see you cast it and watch the terrain suddenly change, I think they'd most likely immediately understand the illusion for what it is.

Worst case advantage on an automatic investigation check.

The floor is lava.

Quote from JayJayPwnsUK >>

The floor is lava.

 Or is it?

I can see the Frost Druids in Time using this to hide things like crevasse's to trap/trick adventurers.

Does the spell disguise the heat of lava?

Since when are roads considered natural?

They form from being traveled alot (unless we are talking cobblestone or paved.)

This seems like an extremely limited, circumstantial spell. Even if you find yourself in a situation where you can use it--say, preparing for an assault--you can only modify the look of natural terrain, so you'd need to be lucky enough to have just the right terrain around you to help with the battle in the first place.

Currently looking over the Mind Flayer Arcanist stat block and I immediately thought this would be a cool spell to make it look like the ground opens up into a huge chasm or lava to stop the PCs from pursuing in a corridor or something but the casting time is a real killer :(

Imagine setting this spell up like a screen beforehand, and then casting prismatic wall behind it so a charging army has no idea they're about to die. Things like that.

This spell was really handy at hiding our caravan of infernal war machines away in Avernus.

How does this interact with malleable illusions?

Quote from MadSkullz >>

Why?

 Because Malleable Illusions lets you reshape anything/everything in the 150ft cube as an action after you've created it