Fort Tourgis Tree - Alderney
This tree is actually on an upper floor of the fort! I think its roots are what keeps the floor from collapsing (the roof has gone a long time ago). According to a book I picked up about Alderney's forts, this was probably the guardroom (notice the slit - which I have learned are called "Musketry Loopholes"). This room would have a door that leads to the "Musketry parapet" above the main gate.
Fort Tourgis, on the Channel Island of Alderney, was built in the 1850's, and is slowly decaying.