Once you've got portals controlled and control fields in place, you don't want them falling to anyone who happens to walk past with some weaponry, right? Right. That's why you will also be picking up Portal Shields, Force Amps and Heat Sinks. You can install these into portals to make them far more formidable. Same goes for higher level resonators, but you might not have immediate access to those, and you can only place four resonators of a certain level at any one portal - so you'll need a variety of ways to defend your portals.