[quote=@Guilty Spark]
[@Letter Bee] Why the difference in difficulty of rituals, as in what mechanics are in place in the world which make the rune circle a necessity for just this ritual type?
[/quote]

Basically, you're building a gateway in-between dimensions, trying to pull together two points of space an infinity apart. Basically, you're doing the equivalent of folding a blanket and then piercing the folded parts with a sword, then trying to pull something from the hole, meaning that Summoning Rituals are difficult, very difficult.

That said, here's an easy exploit I was going to use for my own NPCs: use a cellphone or some other device to create a projection of the ritual circle on a flat surface.