Quote from Lyxen >>

Quote from Sigred>>

The only (relevant) thing that would prevent a creature from attacking while behind an obstruction would be when it is not possible to get line-of-sight on a target from that position: obstruction is too high to lean over, too wide to see around, no holes to see/shoot through, etc. A creature firing from a castle rampart is likely to be considered, in relation to a creature on the ground, as being behind full-cover; that does not prohibit the creature up top from raining arrows down freely. They can see you juuust fine! :P

Joke aside, not necessarily. First, they are not behind full cover when firing. They need to come out, not only to see you, but also to wield whatever weapon they are using. And these weapons usually protrude in front quite a bit. And when they do, they are vulnerable at least to ready actions, and certainly not with full cover at that time.

In addition, you have other elements to consider, like the thickness of the wall, which might prevent you from aiming down. Again, in the movie above, for the elves to be able to shoot downwards at an angle, they had to be quite visible, and vulnerable to orcish crossbows, this part was fairly realistic, and the image above shows you that this gave them around 50% cover at that point in time.

 

That was implied when I said, "Cover is only applicable to the extent that it conceals a target from your sight, at the moment an attack is made, and is based on the percentage of their body that is being obscured."

Nobody attacks from cover. Cover is determined for a creature at the moment that they are being targeted by an attack. If an archer has to adjust from a position of total concealment to make an attack (exposing a portion of themselves in the process), and are immediately targeted by another creature with a readied action, the DM adjudicates cover based on circumstances at that moment. We are on the same page.

Side note: this is also one of the many reasons the advent of the crossbow completely changed the tactics of warfare & sieges. Traditional bows definitely required more physical space to operate correctly, and archers did have to expose more of themselves when firing--as you noted, that often meant actually holding your weapon & upper body over the wall. With crossbows, there suddenly wasn't a need for that much exposure anymore. You only need to jam the front-end of a crossbow into an opening, and can fire without (anywhere near as significantly) exposing your body in the process. The evolution of tactics & weaponry throughout time has followed one basic principle: how can I more effectively attack enemies while making it harder for enemies to effectively attack me?

Lyxen summed it up perfectly with this line: "it relies solely on D&D's sequencing of actions to do a full 6 seconds series of action in a fraction of a second because all the others must have their turn."

It's always worth having a reminder that all these actions are more-or-less occurring simultaneously but to implement that in a tabletop game would be extremely challenging. 

Quote from Chicken_Champ >>

Quote from BKThomson >>

Quote from Sigred >>

BK, when in doubt, just look at the generic creature size:

Creature Size

Each creature takes up a different amount of space. The Size Categories table shows how much space a creature of a particular size controls in combat. Objects sometimes use the same size categories.

Size Categories

Size	Space
Tiny	2 1/2 by 2 1/2 ft.
Small	5 by 5 ft.
Medium	5 by 5 ft.
Large	10 by 10 ft.
Huge	15 by 15 ft.
Gargantuan	20 by 20 ft. or larger

 

5e doesn't place as much emphasis on height as older editions, but creatures (size medium and larger) are typically taller than the dimensions of the cube(s) they actually occupy:

• Tiny, 1-2 ft
• Small, 2-4 ft
• Medium, 4-8 ft
• Large, 8-16 ft
• Huge, 16-32 ft
• Gargantuan, 32-64 ft
• Colossal, 64+ ft

 I knew about the size chart but had not taken account of doing some height math based on that information.  

So, just for conversation, how often in your games do you apply height outside of high jumping movement with creatures?

Just pointing out, for small and medium creatures at least, those height guidelines are actually provided in the PHB/Basic Rules section on Character Height in Chapter 2.

Size

Characters of most races are Medium, a size category including creatures that are roughly 4 to 8 feet tall. Members of a few races are Small (between 2 and 4 feet tall), which means that certain rules of the game affect them differently. The most important of these rules is that Small characters have trouble wielding heavy weapons, as explained in the Equipment section.

 Thanks, Chicken!  This is good to know if height ever really comes into play for a game.  Have you ever applied character height to your game outside of high jump?

Thanks, everyone for making my D&D 101 question into something with more depth to it.  May this thread helps others in their google search for cover and attack information in the days to come.   Yeah, I know, not going to happen but genuine thanks.

For my case and situation, I think the sweet spot may be the 3/4 cover for the goblins behind the crenellations. Full cover while prone feels reasonable and I see Lyxen point about having a disadvantage on attacks that but that type of situation then becomes a house rule because from what I've read, or at least could not find it, RAW would allow Prone to Attack without a penalty applied. 

Fortunately in the encounter, everyone is on the same 'level' playing field but if they were at different heights the cover and attack would have much more complexity when it comes to bonuses and penalties.

Players can hold actions for when they pop out from behind the wall. But it does thwart some play styles like dual-wield, and forces spellcasters to burn their slots if the goblins randomly decide to hang back some rounds.

I'd only use this tactic against experienced players. New players who don't know how to use hold action will just be frustrated.