Open the Research window, select "Physical projectile damage" (>=5) or "Weapon shooting speed" (>=5), and hover your mouse over the rightmost red cross under "Effects". You'll get the message

Unknown key: modifier-description.howitzer-shell-damage-bonus

or

Unknown key: modifier-description.howitzer-shell-shooting-speed-bonus

Please add these keys to your locale file!

Otherwise, I find it irritating that your cannon looks exactly like the gun turret. Wouldn't a cannon image be more appropriate? If you have as little artistic talent as I do and can't come up with your own graphics, could you at least change the color?

Hi Pi-C, thank you for the comments. For the locale stuff, drats, I figured I was missing something but I couldn't see what. I'll get to fixing that ASAP.

For the graphics, I have honestly 0 artistic talent and don't know the factorio animation system enough to do something that would look any good. I am currently looking at how Bob is recoloring the turrets for the multiple levels and will do something similar. I am also looking at increasing the size of the turrets to be 3x3 rather than 2x2 to balance their power compared to smaller turrets.

Hi Pi-C, thank you for the comments. For the locale stuff, drats, I figured I was missing something but I couldn't see what. I'll get to fixing that ASAP.

I didn't know about these keys either -- but fortunately, the game is nice enough to tell you what it needs. So, make a new category in your locale file, put strings for howitzer-*-bonus beneath it, and you should be fine:

[modifier-description]
howitzer-shell-damage-bonus=TEXT
howitzer-shell-shooting-speed-bonus=TEXT

For the graphics, I have honestly 0 artistic talent and don't know the factorio animation system enough to do something that would look any good. I am currently looking at how Bob is recoloring the turrets for the multiple levels and will do something similar.

I recently made a mod for new pipes, and ended up recoloring the images. While working on the mod, I found it very helpful to look at what definitions of entities, recipes etc. contain. You can log it this way:

log(serpent.block(data.raw["ammo-turret"]["howitzer-cannon"]))

The output can be very long because it will contain all attributes of your entity, including inherited values. But if you dig through it, you will get a better understanding of what you'll need. Anyway, there were definitions for picture and animation, and these have a value "tint" -- play around with it to find a nice color, and you have an easy way to change the graphics a bit without even loading a graphics program. :-D

I am also looking at increasing the size of the turrets to be 3x3 rather than 2x2 to balance their power compared to smaller turrets.

Never done that myself, but I guess you could use "scale" to change the image size, if needed.

One more question about your mod: Does it behave like artillery, i.e. does it only fire on spawners, or does it also target individual biters? I'm currently playing with Bio Industries, and that mod provides a "Prototype artillery" (available much earlier than the vanilla artillery, but weaker and with shorter range). I wouldn't really need another turret that provides the same function (i.e. targetting only spawners) with yet another kind of ammo, but if it was possible to target attacking biters with your howitzer, I would have a reason to use it in my current game. :-)

Thanks for the modding help, I'll keep it all in mind.

For the targeting, the Howitzers behave similar to a gun turret, except they are firing larger shells at a longer distance. So they will fire at biters on sight. The extended radius of the shells should help reduce the number of biters making it to your base and soften those that survive for your other turrets.