What properties does pasta have that could be useful in your build? What makes it buckle, flex or snap? If you hold the spaghetti vertical, how does it deal with a load above it? If you place it horizontal, how does it deal with a load above it? How can you make it stronger?
What properties do marshmallows have that could be useful in your build? Will you use your marshmallows as structural nodes? Do you want to split them? Mush them? Stretch them?
What structural elements will define your structure? Will you have a grand entrance, a bridge connecting two areas, or an impressively tall tower?
How can movement make your castle even better? Will the movement be functional or just for fun?
Future Engineers hosts online contests and challenges for K-12 students, including the recent Mars 2020 "Name the Rover" contest. This challenge platform was developed with support from the US Department of Education and all challenges are free for student/classroom participation.