I'm talking about lenses like Canon's 11-24mm f/4L, 14mm F2.8 (both I and II versions), and Nikon's 13mm f/5.6--take a look at their front elements and how they protrude out of the main lens barrel. Just wondering, aside from the lens cap, how does one go about protecting them, especially when in the field, and when travelling?
It will vary from lens to lens, but most manufacturers will provide a lens cap or cover to protect the front element. In the case of the EF 11-24mm f/4 L the lens cap attaches to the integrated, non-removable, lens hood. This provides protection from dust, scratches, and prevents liquid from splashing onto the front of the lens.
As with any other lens, for more protection than a lens cap offers one must consider a well padded protective case. The Tamrac MX5341 M.A.S. Pro 50 Lens Case, shown here with an EF 16-35mm f/2.8 L II, has been reported (see review by hotdog321 posted 5/19/2015) to also fit the EF 11-24mm f/4 L.
I shoot with fisheye lenses a lot.
There really isn't much additional precaution you can take that you wouldn't do for any other lens, minus UV/clear filters. You use a padded bag or lens case, and you use the lens cap. The lens cap is going to be your main point of protection, so you may have to change your habit of removing the cap and stowing it away to having the cap with you, and then only uncapping the lens while you use it, if you're in a situation that could harm the lens.
You become situationally aware. You keep the lens cap at the ready, and you become very sensitive to how close a working distance you can use with your fisheye/ultrawide lens. Because objects may be closer than they appear through the viewfinder, you will almost inevitably early on with the lens while you're learning to use it, end up bumping into a subject with the lens. This is what tends to teach you how to be situationally aware. :)
To protect the front element of my Nikkor 14-24 with protruding front element while actually shooting, I took a high grade sheet of clear plexiglass and dremmeled a disk out of it roughly 1.5" wider in diameter than the front of the lens, took 1/4" strip gaff tape and covered the rough edge of the plexi-disc all the way around, then centered it over the end of the lens and gaff taped it on.
Looks kinda ridiculous, but works like a charm. Only caveats: make sure the lens and homemade filter are completely dust- and hair-free before you tape it on because static makes it hover in the middle and you can't even shake it to one side. Also, it pretty much negates having a lens hood, so lens flare can be a challenge, although it's caused some groovy flare effects that I could work with. It's a temp filter - usually gets trashed because I shoot in the mud a lot and it scuffs up after a while. Hope that helps. Picture of the contraption: