Step 1 Decide Where Your Bridge Will Go and What it Will Cross I'm going to make my hidden bridge cross water for the sake of demonstration, but you can easily use lava or just leave the surrounding area as is. Step 2 Place Pistons For the actual bridge, we're going to use pistons. Place a two block wide row of pistons the length of your bridge one block deep as seen above. Note: If you are interested in a bridge that is not made of pistons, place sticky pistons TWO blocks deep and put your chosen block type on top of them. Step 3 Wire Up Your Bridge with Redstone Remove one block from underneath your pistons the full length of the bridge. Place redstone torches on top of them. This will momentarily activate the pistons. Step 4 Wire Up Your Bridge to a Lever or Other Redstone Activator Here I have wired up my torches to a redstone lever. You can do so without worrying about redstone being visible by running the redstone wire that is connected to your torches up and underneath the block that is your lever, or that the other redstone activator is on. Hope this Helps
How is it hidden? Needs to be built one block further down below water surface. Good job though looks pretty cool