I don't know if I said that correctly. I can't find the words to say, but I hope you guys get my point. I'm currently making a Low Poly cave for my game and been making a surface by just using a Plane. However with the reference picture below, I wanted to cut the excess plane outside of the bulky walls and only keep the inside. I've been fiddling with the Bool Modifier, but I guess that won't work. Any help will be appreciated! Thank you!
A tool which will cut your mesh in any case regardless of topology is Bisect.
Let's say we want to cut this:
Got to Edit Mode, Side View, and activate the Bisect Tool from the Tool Shelf:
Drag a line across your model, it will be the cutline projected from the view you're looking from through the whole model. The cut will be made, and the cutline selected:
Now, select one of the newly created vertices, and run the Select Side of Active Operator from the menu:
Tweak the operator settings to fit your situation:
Now, deselect the parts you want to keep, and delete the faces afterwards:
The whole process is a bit lengthy, because you have both faces you want to keep and faces you'd like to delete. If you manage to separate out the faces to cut beforehand, you can tweak the Bisect settings directly, as it has an option to remove either top or bottom half of the mesh.
If the boolean modifier doesn't work (but it would be my first try to do what you want), you can try to use the knife project (view dependent).
You have also the tool Intersect (Boolean), which required to have your 2 meshes joined and to be in edit mode. Select your plane, press space and search Intersect. Options are like usually on the bottom of the Tool Shelf.