Blocks are used to delay monsters in their attack on your base, giving your towers more time to kill the attacking monsters. Blocks are highly recommended as a player reaches level 26 and higher, as your base will be more vunerable to stronger players. Unfortunately, even the heaviest walled defence will fall. Well defended level 2-3 Eye-ras with their explosive effect will damage or destroy several blocks at once. So it is advisable to place higher level Towers around walls as higher level Eye-ras have relatively faster speed than most monsters and decent health. Even so, Wormzers or King Wormzers can still evade booby traps and walls, thus making your base more compact is a better option as multiple Towers can easily attack Wormzers/King Wormzers.
After you have constructed your wall, sometimes you will have a few extra blocks laying around. Don't waste them though. A good way to use these blocks is to place them at the outer edge of your map, forcing an attacking player to place their monsters further out or in a differrent place. Remember though that the diameter of the attackers circle is about 7 blocks so don't put the blocks out too far.
The strategy also works well against eye-ras and will force an attacking player to use 2 Eye-ras as they go for the nearest wall. Eye-ras take up 60 housing per eye-ra and take 25 minutes to produce. This is a lot of space for a low or mid level attacker and could make them change their mind about attacking you.
Another good way to use leftovers is to scatter them all over the outside or inside of your base (Don't put them too close together or eye-ras will destroy them all) so attackers will have to fling their monsters far out and give your towers more time to defend. Also, this will force attackers to use multiple Eye-ra to attack walls, as they will first attack the extra blocks scattered around.
A good way to use blocks is with pathing because a player needs only (1 with level 6 Eye-ra) 1 Eye-Ra to destroy a Wooden Block , (1 with level 6 eye-ra) 2 Eye-Ra to destroy a Stone Wall , (1 with level 6 eye-ra) 5 Eye-Ra to destroy a Gold Diamond Wall and (2 with level 6 eye-ra)7 Eye-Ra to destroy a Black Diamond Wall.
NOTE: blocks are better used as pathing