Warlight is, of course, a simple game with complex possibilities (like risk, chess, etc.). The original risk, and the default warlight maps have territories with political boundaries, and it makes sense for all territories to be equal. But Warlight also allows for custom maps, and there are many maps available that include historically (and fictionally) defensible areas. Think of the 300 spartans at Thermoplyae, of the Panshir valley in Afghanistan that thwarted the Russian occupation, of castle sieges, etc.

To me it makes sense that Warlight could really benefit by adding this simple feature. A territory could be set to give a defensive advantage to the armies within it, similar to how you can set the offense and defense attack strengths of the overall game. Perhaps it would just be a flag with a default defensive bonus, or perhaps it would be a scale of bonuses to be chosen by the map maker.

In developing a custom fictional-historical map, I found myself really wishing that this was possible. I think it would be an excellent feature for maps that include famous sieges or defensible locations.

I believe that there is a User Voice for this

Think of defensive territories not as super bonuses to a single spot, but as a single territory that can be used to defend many. For instance Hawaii on any of the world maps. Defending Hawaii protects lots of places and forces armies to go way around in order to attack bonuses. Contrast that with say, one of the northern territories in Central Russia. Holding that territory won't allow you to defend anything else, and armies can walk right through.

300 worked because it was a narrow pass, forcing the huge army to wait until the 300 were defeated before moving on.

As for making them more defensive, two cards already exist to do so, Abandon and Blockade.

There are a lot of options I'd like to see for singleplayer maps, as the multiplayer options rarely provide what would be nice for singleplayer. As you said, a defensive bonus of a sort. For example, France in World War II. The Germans couldn't really break through their defensive line, so had to go through the Netherlands and Belgium. But there's no way you can recreate this option in the current state of Warlight. Being allowed to change the defensive/offensive bonus of a territory should have no impact on multiplayer, as it would make for a custom scenario.

Also, more official singleplayer maps.