To make a fort that is actually good at being a fort is tricky business. More often that not that fort will just be one big, juicy target too easilly approached from blind angles and overrun. Also under-mining them and destroying large portions of their battlements in little time is also quite easy. To be frank, forts are redundant.
Trenches, if aptly placed, are great battle stations. In my eperience, they should be placed on hillsides where they begin to incline downards. Very useful as when you run over the open ground that hilltops oftenly are comprised of, you can leap down into the trench and get a good view of the stretch of land ahead. And by digging the trenches just a couple of blocks below the beginning of the slope, the other team will have a hard time using it as a fortification against you. The slope itself provides a good obstacle that will slow down the enemy if they should try to assault you from that direction. They also make a good starting point for building foxholes further down the slope. Trenches are of course vulnerable to grenades, flanking and rear attacks. Thus it's always a good idea to make the trench in an irregular pattern, not just one long and straight ditch.
Pillboxes are in the grey-zone of usefulness. They are highly conspicuous and tend to draw the enemy towards them. So if you're wanting a good sniping spot, pillboxes are not for you. They are supposed to be placed in the way of enemy routes and to recieve a constant barrage of bullets. Unless you make very large windows, witch kind of defeats the purpose of a pillbox, your field of view will be limited. Generally, go with either 1x1 if the area ahead of it is flat, and 1x2 if it's jutting downwards. But always keep an eye on the entrance, since they are easilly circumvented. Making the wall two blocks thick on the sides might also prove beneficial, giving you some extra cover should things get really heated. Repairs can always be made at a later time. Lastly, something I've started doing is putting a coloured square on the roof of my pillboxes equal to the size of the intel square on the map. This will inevitably draw the enemy towards you, so be prepared. Can be undermined, so listen for diggers. Grenades are always a threat here as well, as the space inside is usually limited.
Crennelated walls and "Eagle's Nests" are something of a marriage between trenches and forts. I'm not sure I've ever found them very useful. Oftenly they are overrun by enemies wich turn them against you. As they tend to be placed on hilltops just before the incline, walls work equally well despite wich side of it you are standing on. But they can still be good for giving you a sheltered vantage point over the area ahead. Building these can be very dangerous, especially if you want them overlooking enemy territory.
Fox holes is where the money is at. In AoS they're like subterranean trenches, opening up with single block holes here and there. They are extremely hard to spot. Generally you won't notice them until the tracer-fire is flingin' around your blocky head. They are quick to build, and in my opinion unrivaled as a offensive fortification. It can be a good idea to place them ahead on the flank of a trench, wall or fort, looking sideways over the ground infront of said fortification. This will provide a veritable shooting range as the enemy tries to advance towards the battlements, oblivious to your little peepy-hole way over on that hill no one cares about. Provided you know how to aim, you can kill dozens of people before they figure your position out. They are easilly defended and easilly escaped through glorious digging.
Tunnels are ample subbteranean highways, just keep your eyes peeled for any travellers in the opposite direction.
Towers is what you build if you want to be shot at from eleven different directions at once.
I'd also like to add my thoughts on the best intel defence: A pillar 2x2 thick, no more, since that gives the enemy a tube to climb through unseen. Make it decently high, and then make as many support pillars as you wish, preferably 2x2 as well that you erect next to the intel pillar and then connect them at the top. This is far from fail-proof but it will take some time getting to the intel for the other team while your own guys spawn nearby and can be directed to defend it.
I really love this game.