The Sand Fortress was a structure that originally surrounded the Desert Glass in "Steven's Lion", its only appearance so far, before the Crystal Gems captured it. When the Desert Glass was accidentally thrown onto the beach in front of the Crystal Temple, it started rebuilding the fortress.
In "Steven's Lion", the fortress was originally seen in the Desert, where its pillars were constantly being built, demolished and reconstructed by the malfunctioning Desert Glass. After the Gems secured the Desert Glass, the fortress crumbled back into sand.
When Steven threw the Desert Glass out the window it landed on the beach and started rebuilding the fortress. This time around, the fortress was much more deadly. It was shown that the fortress was nearly indestructible, seeing that Garnet could not break it when she tried to punch the walls of the aforementioned fortress. Lion was able to break through the walls with his concussive roar attack allowing Steven to secure the Desert Glass, after which the fortress once again crumbled.