This is a bit of a mystery, I haven't figured out what the building was originally. The construction is robust and includes some fancy touches, but I don't know what the ruin was originally for. It's the only structure, or ruin, for quite some distance, in an area along the Little Manatee River.
As a technical side note, this didn't get into the recent "walk in the woods" post because there was unacceptable sensor fringing in the branches against the bright overcast sky. But today I was experimenting with the new and vastly improved "defringe" tools in ACR version 7.x (part of PSCS6). It occurred to me to go look up this file. The new tools easily changed false-color branches into believable-looking bright highlight areas.