I'm trying to come up with a way to pathfind on multiple levels. See image:
The orange walls would be considered a ladder, connecting you to the upper levels.
At first I thought of looking at it as though it were top down 2d, but I also need to be able to go under things at some point. How can I compute these paths?
Pathfinding in general is a graph searching problem. Since you are doing a grid based solution, Unity's nav mesh might not be what you want. You should look into A* search as there's many resources out there for it. Your graph has nodes and directional edges. Wherever your character can stand and occupy space is a node (each block face in your 3D world) and each edge is a link showing the character can move from one block face to the next. Assuming you know A* given your mention of 2D pathfinding, your problem boils down to, how can I get a graph representation of my game world. Assuming you have some data structure holding all the data of where blocks are placed (a 3D array with true/false for example) you can figure out how they are connected. 2 blocks adjacent to eachother on a flat surface means their top faces are linked in your navigation graph. A block placed above an adjacent block (i.e. 1 to the right and one up of another block) means the other block's top face connects to that block's side face IF there's a ladder. You can continue this logic until you have a script that builds a graph representation of your world. Then just run A*
