This is a step by step instruction on how to create a retracting lava bridge(video tutorial at bottom)

Step 1: Step 1: Create World

To start you are going to need to create a world, Name it what ever you like make sure it is set to creative. you also need to make sure the world is also set to super flat. Also changing the 3rd number in the present section to 100 it will be set to 2 automatically.

Step 2: Step 2: Materials

You want to collect
1.Piston

2.Lever

3.Redstone

4.Redstone Repeater

5.Gravel/Sand(either will do, depends on look your going for)

6.Stone

7.Dirt (To fill up gaps, material will change if you decide to do it in a different location

8.Lava Bucket

Step 3: Step 3: Hole

The hole that i created is 10 blocks in length, 7 in width and 5 in depth, yours may vary in length and width depending on the size you want

Step 4: Step 4: Placement of Redstone and Pistons

Sorry for not doing a more in depth instruction but mainly just follow the look of the pictures or watch the video, but once complete do a quick test to see if yours works

Step 5: Step 5: Filling and Test

Plain and simple place stone, grass or any block really except wood 1 layer down from top, make sure not to interrupt the Redstone circuit. Only place gravel or sand over pistons, fill up area with lava like so on the picture and pull the lever to test. Enjoy your lava Bridge!!!.

You can alter the size in to fit your needs a perfect use of it would be for a castle moat.