In Defoe's Robinson Crusoe, why does Crusoe build his fortress with so many defensive features—from what is he defending himself?
I believe that in Daniel Defoe's Robinson Crusoe, with the construction of his fortress (or "castle" as he calls it), Crusoe is concerned with several things. Completely out of his element, Crusoe discovers how little he knows about the world (building things, farming, hunting, etc.) and how much he must learn if he wishes to survive. He overcomes many obstacles through investigation, analysis, and sometimes by accident. He analyzes the best place to build his "fortress."
It will be recalled that in Defoe's tale Crusoe decides to build his permanent dwelling on a hill with a view of the sea. "I found a little plain on the side of a rising hill, whose front towards this little plain was steep as a house-side, so that nothing could come down upon me from the top...
...though, as it appeared afterwards, there was no need of all this caution from the enemies that I apprehended danger from."
Crusoe goes to great lengths to deal with potential threats. Things that Crusoe is unaware of are the dangers he could face on the island: is it deserted? could he be attacked by wild animals? He is also confronted with the unknown conditions of the island's weather, and even his own lack of knowledge, in general.
I believe that there is also a psychological element to his work as well. The building of a "castle" gives Crusoe structure and purpose in each day. With each achievement, he may experience a sense of humility. E.g. when he makes clay pots, he is honest with himself:
I must needs say as to the shapes of them, they were very indifferent, as any one may suppose, when I had no way of making them but as the children make dirt pies...
Crusoe does not worry for things that he makes crudely (like a suit of clothing made from animal furs he has cured—they are functional); there are other times he describes his satisfaction in his accomplishments, as with creating a "household" in his castle, including: a parrot, a dog, a cat, and a very tame goat.
One day, Crusoe discovers a human footprint in the sand—after being on the island for many years.
It happened one day, about noon, going towards my boat, I was exceedingly surprised with the print of a man’s naked foot on the shore, which was very plain to be seen on the sand. I stood like one thunderstruck, or as if I had seen an apparition...
...but after innumerable fluttering thoughts, like a man perfectly confused and out of myself, I came home to my fortification, not feeling, as we say, the ground I went on, but terrified to the last degree, looking behind me at every two or three steps, mistaking every bush and tree, and fancying every stump at a distance to be a man.
In light of the foot print, Crusoe works to improve his fortress even more. The fortress protects Crusoe physically, and provides mental and emotional stability as well—addressing concerns he has about what he knows and those things he is not certain of. He accomplishes a great deal in building a safe haven for himself on the island.