When setting out to build a sandcastle, you must first buy yourself a pail and spade. These can be purchased from Paddywhack's Knicknacks on the boardwalk, conveniently located near the beach where you have the second thing you need for sandcastle building...sand!
After finding a suitable spot along the beach (you'll know if it is suitable if it lets you fill your pail), hold your pail in one hand and your spade in the other and DIG. Now that your pail is full of sand, DUMP the sand out into a pile (you need two hands to do this so you'll need to put the spade away). The more pails you fill, the larger the pile of sand you can make, and therefore the larger the sandcastle. To make bigger piles from the tiny piles you need to PUSH PILE TO OTHER PILE. You may find that some piles are too big for you to work with, wait until you learn a bit more in gadgeterring before you try castles that big again. If you find your pile is too large, give it a swift KICK to bring it down to size. Here are how many buckets full you need for each size...
Tiny - 1 bucket
Small - 2 buckets
Normal - 3 buckets
Large - 4 buckets
Enormous - 5 buckets or more
Now after you have your sand all piled together, you're ready to work on the sandcastle itself. First you want to PAT your pile, make sure it is your pile and not somebody else's. If you don't pat the pile before carving it can fall apart. After you have the pile nice and firmly patted down you want to CARVE PILE, then you kinda just sit back and watch your sandcastle take shape as you add moats, towers, crenallations, roads, and any number of other things. You won't see the details on the finished product, so be sure to watch as you build to get a feel for how your castle looks.
There are five different classifications of sandcastles judged heavily on your gadgeteering skill and a bit on your art skill. Some say coordination plays a part, but I have tested it extensively and coordination did absolutely nothing for building a better sandcastle. The classifications are as follows, from worst to best...primitive, crude, plain, fancy, and fanciful.
After building your castle you can sit back and watch the local wildlife play around it until it is destroyed. There are several ways to destroy a sandcastle, either by yourself or by some outside force of "nature". Leaning on a sandcastle will knock it over, as will pushing and kicking (other players will not be able to harm your castle). If you build anything normal or under, dogs will come by and roll around on it after a while and destroy it. If you build anything fancy or better then a bully comes and kicks it over then proceeds to attack you.
Now that you have all the basics, grab your spades and pails and go at it! The best place for building sandcastles as it stands now is The Boardwalk just south of Piraeus, this is also where the spade and pail shop is.