Welcome, and thanks for checking in, Agent.
This is a summary of Portal Mods that are used to defend a Portal against attack by enemy Agents. They include Portal Shields, Turrets, and Force Amps.
First of all, Portal Mods are classified by type and rarity. Some Mods have multiple rarities while others do not, but the base rule is that a higher rarity Mod has a more powerful effect than a Mod of a lower rarity. The only Offensive/Defensive Portal Mods with multiple rarities are Portal Shields, which may be found in Common, Rare, and Very Rare grades. The particulars of each will be described below. Turrets and Force Amps are only available in Rare grades. Note that all mods require XM to be deployed, and that a Mod of a higher rarity requires a significantly larger amount of XM. Furthermore, a single Agent may only place 2 Mods on a Portal.
With that being said, let's look into the specifics of each Portal Mod.
First, let's examine the most commonly used type of Mod, the Portal Shields. They are used to reduce incoming damage from enemy XMPs, which allows Resonators to last longer when being attacked. In addition, they make it more difficult to destroy other Portal Mods. Shields (and other Mods) have an innate "Stickiness" value that causes them to be easier or more difficult to destroy, based on their rarity. The higher the Stickiness, the harder it is to destroy the Portal Shield.
As with other Portal Mods with multiple Rarities, Portal Shields that are of the Common rarity are green. They provide a Mitigation of 30 (reducing incoming damage by 30%). However, they possess a Stickiness of 0, meaning that they will be destroyed after only a few hits from XMPs, and almost immediately from Ultra Strikes.
A Common Portal Shield is shown below.
Next is a Portal Shield of the Rare rarity. Rare Portal Shields are purple, and provide a Portal with a Mitigation of 40 (reducing incoming damage by 40%). They also have a Stickiness value of 15, meaning that they possess some resistance to Ultra Strikes, and decent resistance to XMPs.
Next, a Portal Shield of the Very Rare rarity is pink in color. As the 2nd most powerful of the four Portal Shields, Very Rare Portal Shields provide a respectable Mitigation of 60 (reducing incoming damage by 60%). They also possess a Stickiness value of 40, making them difficult to remove with XMPs, and somewhat challenging with Ultra Strikes.
The last type of Portal Shield is the AXA Shield. These highly resilient shields provide a massive Mitigation of 70 (reducing incoming damage by 70%). While this is not much more than the Very Rare Portal Shield, the difference is that AXA Shields possess an unparalleled Stickiness of 70, making them nearly impossible to remove with regular XMPs, and very difficult with Ultra Strikes. These shields are the epitome of a well-defended portal.
Moving on, let's look into the two types of offensive Portal Mods.
First, we come to Force Amps. Force Amps are a relatively simple Portal Mod. They exist only of the Rare variety, and double the damage dealt by a Portal's attacks against attacking enemy Agents.
A Force Amp is a powerful Mod to have on a well-defended Portal, because it causes an Agent to lose twice as much XM every time they attack the Portal, which can make it very costly to their reserves if they wish to continue attacking the Portal.
The other of the offensive Portal Mods is the Turret. The advantage of a Turret is that it makes a Portal guaranteed to attack an enemy Agent when they attempt to Hack or Attack the portal with XMPs.
Using a Turret is wise when equipping a Portal with a Force Amp because it allows the Force Amp to be effective more often. It is, of course, also possible to take the chance of the Portal not attacking every time, but dealing significantly more damage by placing multiple Force Amps without a Turret. Both are viable options, but the final decision is up the the Agent.
Since the update to Shielding strength, even placing two Common Portal Shields on a Portal can make a large difference in its survivability, and with the relative frequency with which they can be acquired, it is an inexpensive way to ensure Portal longevity. Remember that because Portal Shields also protect other Mods from being destroyed, it is important to use them frequently. For example, a Portal armed with a Turret and 3 Force Amps would certainly deal a great deal of damage to enemy Agents, but after only a small number of XMPs, they would be destroyed. Thus, it is up to the Agent to find a balance of Mods that they believe to be the most effective.
This concludes the discussion of the basic game mechanics of Offensive/Defensive Mods that are available for use in Ingress. Check back to this page as often as needed to answer any questions about these Mods, or feel free to email me at firstname.lastname@example.org, or shoot me a message in game at @EliteFerrex with any questions. I hope this information serves you well.
Good luck out there--it's time to move, Agent!