A good castle wall in crab battles should be at least 3 levels high, and have 3 layers!
Heres a tip on beating crab battles:
do not use the 1st, or the last.
1. the first cannon is not good enough.
2.the last takes to long to get
3. the second one is not to expensive, and works well!
hope this helps =)
To get the metal detector on crab battles you have to beat the first wave of crabs.
NOTE: you can get it only once after every wave! =0