
The blue portal can of course be anywhere else, ideally far away and well defended

Idea behind it: enemy wants to grab flag, but can't touch it because he gets teleported away by the portal first.
To set up an almost invulnerable portal flag defense, you need a deployer and helpful team mates:
- Deployer puts red portal at flag, blue portal elsewhere, and runs away (e.g. hides, or stacks up health)
- Helpful team mates regularily use the portal (e.g. by jumping in the red portal, or shooting a rocket/electro/laser/grenade/hagar at the red portal)
Why? Easy. Portals can time out, and can be destroyed by shooting the BLUE portal. But USING them in any way (e.g. teleporting yourself or some projectile through it) refreshes their health and timer. So regularily "feeding" a portal keeps it alive.
Portals also go away IMMEDIATELY if their owner (the one who created it) dies. That's why the deployer should go to some safer place.
A clear abuse of the behaviour of portals, but I like it. A nice strategic element

To break the portal flag defense, the enemy flag carrier has to destroy either the blue portal before someone feeds it again (maybe kill these guys first, note that shotgun can't feed portals), or kill the portal's owner (who may be anywhere).
The bad part is, the flag carrier has to do it TOO. So when the helpful team mates see their flag carrier, they start shooting the BLUE portal instead of the RED one with all they have, to destroy the portal flag defense and make their flag carrier cap. The flag carrier should of course shoot at the portal too.
NOTE however: portals are only vulnerable to explosions, Laser, Mortar, Electro, Crylink, Hagar, Rocket Launcher. The Shotgun, MG and Nex can NOT harm or feed a sophisticated high-tech device like a portal!