The Howitzer is a location in-game. The other locations can be found on the Map page.
The Howitzer is a minor location. There's nothing important there. On the other hand, it provides quite a good view of the surrounding area...
The Howitzer is exceptionally difficult to fortify, since it literally is just a howitzer right next to a small bunker house. There are no pre-existing fortifications in place to protect you from zombies. However, despite having no fortifications, the howitzer is actually very good as protection. Standing on the tip of howitzer means that a single zombie can only get to you by walking down the howitzer. The only thing that can reach you, other than a zombie climbing the base, is a horde that collects under you and slowly builds up height.
Because of the howitzer's height, it also serves as a sniper perch, of sorts. Below the howitzer is a wide open field, which makes zombies target dummies.