to add defensive fortifications to a site to withstand attack — "fortify the camp"
to make a structure or organization stronger — as in "fortify the wall"
to inspire emotional and mental strength — as in "fortify her spirits"
to strengthen an argument — as in "fortify our argument"
to add nutrients to increase the healthfulness — as in "bread fortified with folic acid and iron"
to add alcohol to make a drink stronger and/or less prone to spoiling — as in "port, sherry, madeira, and other fortified wines"
I endeavoured to crush these fears and to fortify myself for the trial which in a few months I resolved to undergo; and sometimes I allowed my thoughts, unchecked by reason, to ramble in the fields of Paradise, and dared to fancy amiable and lovely creatures sympathizing with my feelings and cheering my gloom; their angelic countenances breathed smiles of consolation.
Chapter 15 (49% in)
There are no more uses of "fortify" in Frankenstein.