The castillo is a tall, wobbly wooden structure affixed with fireworks that spin and spew sparks and colors in stages from bottom to top. It's a really impressive show to see, even if you are a gringo (and can put aside your inborn fear for everyone's lives).
We saw a castillo last night in front a the church that was celebrating its 35th anniversary. Below are my photos, although the spectarcularness is hard to capture on film at night.
It is set up in the middle of electric lines. No one seems too worried.
The castillo rains sparks all over the cheering crowd.
Kids run under the sparks, trying to get rained on.
This kid tries to get a better view by climbing the ladder on a truck (labeled "Hazardous Materials").
The grand finale! The corona (the top part) ignites in a glorious plume, and suddenly...
...the corona detaches completely and shoots straight up into the sky!
It extinguishes mid-air and lands near a car on the side street.
To witness a castillo, pay attention to when your local churches hold their fiestas patronales, or any other town festivals. You'll probably find lots of castillos during the Christmas season too.