fortress (n.)

mid-15c., "fortified place, stronghold," from Old French fort "fort, fortress; strong man," noun use of adjective meaning "strong, stout, sturdy; hard, severe, difficult; hard to understand; dreadful, terrible; fortified" (10c.), from Latin fortis "strong, mighty; firm, steadfast; brave, spirited," from Old Latin forctus, which is of unknown etymology. Possibly from PIE root *bhergh- (2) "high, elevated," with derivatives referring to hills and hill-forts, or possibly from *dher- "to hold firmly, support." Figurative use of hold the fort attested from 1590s.

early 14c., "harsh or severe treatment," from Old French duresse, durece, from Latin duritia "hardness," from durus "hard," from PIE *dru-ro-, suffixed variant form of root *deru- "be firm, solid, steadfast." For Old French -esse, compare fortress. Sense of "coercion, compulsion" is from early 15c.; in law, "actual or apprehended physical constraint so great as to amount to coercion" (early 15c.).