One of the best defensive summoner's spell that I frequently use is fortify (usually with a heal). Fortify makes all ally towers invulnerable with a 100% increase in attack rate and added splash damage. Even if it is a good spell, if you do not know when to use it, it is pretty useless. In this pro tip, I will give situations which would be a good fit for using fortify.
1. The other team is pushing your tower as 2-5 and your team is ready to defend: Let them crowd around the tower first then have your team use all of your aoe spells and 1 fortify (do not stack them) near simultaneously. Ie: Gangplanks Cannon Barrage, Fiddle Stick's Crowstorm, Amumu's Curse of the Sad Mummy, Janna's Howling Gale, Malphite's Unstoppable Force, and etc. Using your AoE spells will clear all the creeps and allow the tower to dominate the enemies. The two most important part of this is 1. initiation and 2. crowd control. You want to do everything near simultaneous as possible with the exception of stuns to avoid stun stacking (Amumu's ult & Malphites ult). To crowd control is to keep your enemies around the tower so they can take more hits from both your team and your tower.
2. You want to use fortify when people are tower diving. I see too often where people tower dive and get away with very little hit point and a teammate has a fortify up. This would in practice only work if you have teamspeak or ventrilo because calling out a fortify will take too long by typing.
3. Heal Baiting. My most favorite of them all. I generally gauge my hp and will know how many hits I can take before I die. Sometimes people will dive by flash/casting or ghost/casting and take a couple of hits before getting out with a kill. Its a little difficult by it is possible that you can heal as a bullet,spell, or arrow flys at you if your time is good. If getting tower dive happens frequently to you, try saving that heal and fort for crucial moments and it might save you at the same time give you a couple extra kills.
4. You are laning Rammus against a melee hero:
One of the thing I like to do when I play rammus is use puncturing taunt and pull people into tower then fortify. I will stun them when the taunt wears off. An important thing to remember that it only works on melee characters and is best when they run up to last hit a creep just out of or in tower range.
5. You are Alistar. One of the most common technique when using Allistar is to flash in stun your opponent, then push them into tower range. Its imporant that the hero is just out of tower range and the tower is not attacking anything because if it is attacking creeps when you push the champion in it won't switch targets.
WHEN NOT TO USE FORTIFY.
If tower is going to go down anyways. For example your team is scattered and they are going for a base push and the 2nd tower is already low. I suggest that you let the tower fall while you group up and prepare for a fight inside the base with fortify.