Collapsing castle

Castle I built has developed cracks
May turn me naked where protection lacks
For any time the cracks on walls
Will destroy the castle and all effort falls

Unaware I was while building
Feeble castle I am developing
Now I discover skeleton was right
Weak bricks I chose and they failed to hold tight

Inside castle effortless was flow
Once it collapses I will face the blow
Like a tranquil stream in all its quiet
Encounters a cliff and falls down the height

There lies an ocean down the cliff
Swimmers here survive grappling tough tiff
Courage I should harness, swimming I should learn
Again build a castle very stiff,secure and stern.