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.Eman, whats your source of information that fortified units receive ANY defensive bonus for being fortified? The manual certainly doesn't say that.
BTW I have no idea what "(viz. 0.75)" means.
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.