Remember when you were a kid at the beach and tried to build a sand castle as the tide came in? Welcome to Crabs and Castles.
Essentially you dig and fill in squares to build a sand castle to protect your flag. Meanwhile waves roll in, which will disrupt your construction and ultimately carry the flag out to sea.
Use the shovel to dig up sand, and use the bucket to dump it back on the ground. BUT WATCH OUT! When a crab comes out, click it till it goes away!
Space Bar = Toggle between shovel and bucket
Left Click = Action (Dig/Dump/Hit)