Defender Saves the Day is the thirteenth mission in WorldBuilder.
Build a Steamshovel, then pick up the plans for the Defender. Build two from the piles of bricks laying around. Be sure to have your Defenders ready to fight the Crab on the other side before bridging the gap (you can deal the most damage to Crabs and save more health for your Defenders by having both of them work together against them). You'll also find that Crabs will drop fresh Energy Bricks upon being disassembled. Be sure to replace your Defenders' energy bricks if necessary.
Grab the plans for the Forklift that are near the original goal location, then use your Steamshovel to bridge the gap to the goal. Afterwards, move it to one of the piles of red bricks left behind by the disassembled Crabs, take it apart, and use the parts to build a Forklift.