Can I just say that the first thing I think of is "White Castle"? ((Tasty!))
Alas, are you:
I am a castle with plain white walls
It's a box that is traditionally white (nowadays I guess they're silver).
but on my inside are no halls.
Well... it ain't a castle.
My creator laid me down with care -
You don't put toasters next to your stove, in a microwave, or in a bathtub, for instance.
all smooth corners, never square.
I don't know about you guys, but our white old toaster is rounded on its edges. This seems to be the case with a lot of toaster designs... I don't know why.
Inside my door you must create,
Create some toast. If it has a door though, it might be one of those fancy toasters. You know the ones.
You'll find my prize, my gold estate.
Toast 'em till they are golden brown... mmmm
And though it cannot walk, it runs -
Would you count them popping out at the timer "running"? They're running from the heat! ha. ha. Or the timer "runs" out of time- ticking down until it pops the toast out.
Unless you burn it 'til it's done.
You can burn toast easily. Also relates to above, "running" out of time and all-the-while burning that bread.
What am I, and what's my prize?
Toaster & Toast