a fortified building, esp one built in former times for defence, or any strongly fortified place maintained for defence
hold the fort
to take charge of something for somebody or deputize temporarily
to cope with problems for somebody, or look after their work, while they are temporarily absent
[with allusion to a signal sent by General Sherman during the American Civil War urging General Corse to hold the fort at Allatoona Pass until reinforcements arrived; the message, misquoted as hold the fort, for I am coming, was later incorporated in a popular gospel song][Middle English forte via French from Latin fortis strong]