If one wishes their edifice to weather the external elements --both the heat of the summer and the cold of the winter-- the key is fortification. For what better method is there for ensuring the safety of ones foundations from any storm that may come? Yet without it, ones sanctuary can become filled with all the evil that may lurk from without.
[It's said that Rabbi Yisrael Lipkin (of Salantai) was approached by a man who had only half an hour a day to study (Torah), and wanted to know what subject to delve into. Rabbi Yisrael answered that he should study "מוסר", for it will inspire him to make more time for himself to study other things.]