Thoureau wanted to poit to the fact that following traditions limits us and deprives us from developing our imgination. Thoreau, like other romantic authors, does not agree with tradition. He thinks it prevents people from finding true greatness and blocks their path to spirituality. "Castles in the air" are simply illusions by which people persuade themselves in something that is not true, in Thoureau's opinion.
usually if you look it up they will give you essays on it. which might be more helpful
Land surrounding the Panama Canal is very fertile. The warm tropical weather is good for growing things. The original river and later the building of canal made trading very convenient. So there are a lot of industries developed in that region.