The triggered ability is mandatory. If you don’t have a creature to target, you must target another player’s creature, if possible.

The second ability triggers whenever the fortified land becomes tapped, not just when it’s tapped for mana.

Fortification is to lands what Equipment is to creatures. Except for the difference regarding what kind of permanent it can affect, Fortifications and Equipment follow the same rules.

If Darksteel Garrison and the fortified land would be destroyed at the same time, only Darksteel Garrison is destroyed.