The End of an Epoch
Centuries ago the lands were ruled by powerful wizards; they were so powerful many of them chose to rule from on high, literally. They ripped their own castle, keep, or citadel magically from the earth and hurtled it into the skies to better protect their people.
On one single day they all fell. Despite dozens of colorful rumors, no one knows for certain how or why. More than one of those rumors are rooted in the fact that the day the fortresses fell was the autumnal equinox. Most of the Wizards survived the destruction of their strongholds, so they retained their power overall, but this was the beginning of the end for their rule.
A few of the fortresses survived somewhat intact. Those that did became popular stomping grounds for the adventurous. Some became so popular entire cities, like Fortress Falls sprang up around them.