If you look in the Civilopedia (Map Screen, top left, middle button) under Defender Combat Bonus, you will see the 25% bonus for fortified units. You will also find the Fortification Bonus if you look at the Combat Calculator utility programs here on CFC Forums. For example, if your defender has a defensive value of 3 (e.g. Pikeman), you will get a fortification bonus of 25% of 3 = 0.75. BTW, viz means To wit; that is; namely.