I find this is very interesting topic :)
Well.. there is a super nice game "Airscape : The Fall of Gravity" which made from Construct 2, the "curved surface" also can be found there, and I think it is similar to Alto's surface..
Make a solid surface in typical/basic platformer game is easy using Construct, but I stil cannot figure it out how to make a solid "curved surface",
I have no idea how to define the collision polygon perfectly to look like curve since there is warning not to make more than 8 collision point,
make the player character move smoothly on curved surface could be challenging too
I also curious about how to make the hole on terrain which caused by projectile shoot..