I'm going to add something to this. Colosseum walls. They do not always snap together when you edit the coordinates. Occasionally when you are trying to build a floor, there will be a bump, and you won't be able to get rid of the bump by editing snaps or coords. So what do you do?
First, you spawn a grid (or two), then edit the coords of the grid. Then, you set the Colosseum walls to "Physics: Normal" (press X and look for physics, it should be set to phased). Fixed also works, but not as well as normal. Then you set the Colosseum walls on top of the grid and edit the coords of them so they are nice and even with each other. Then, you go in and set the physics to "fixed", if you were using normal. Don't set them to phased, because they may glitch. Your Colosseum floor should now be done
This also works with other hard to snap items.