I know the first thing when making a good defense for the humans is to make a half-moon entrance so they can shoot with a lot of units.
But after that I don't know how to make good tunnels, first & second lines, multiple paths, room centrals, the overall structure.
Plus I don't fully know which tiles you can see through, I ended up with some one-tile thick walls which I could see through the edges, especially at the corners. I remember that I placed higher ground tiles next to unpassable tiles.
None.