I've gotten a few questions about going over maps and possibly discussing strategies. In this post I'll cover some basic competitive strategies for the map, and then some pubbing strategies.
First, some little-known facts about the koth (king of the hill) format:
1. The point doesn't unlock for the first few seconds of the map. Unless you're trying to rush some sort of engineer setup or are extremely confident that you won't die from spam, there's no point in running out onto the center of the map, out of cover.
2. Once your team caps the mid point, your team's respawn times will be significantly longer than the opposing teams. Think 7 seconds compared to 15.
3. The midpoint is extremely open. On all koth maps, the midpoint has little to no cover, and is where the majority of the kills occur on each map. Unless you have a legitimate, conscious reason to be out of cover, try not to be. Especially on a map like koth_viaduct with such good sniper sightlines, you'll get killed pretty quickly.
Now, using those three basic tips, you can get a pretty good idea of how strategies for this map work: stay in cover. Abuse map design. Heck, those are pretty much good strats for every map.
For 6v6 play, you don't have to worry about random spy or semi-random events (mini sentries in odd locations, spies, etc) as much. However, you're going up against people who know how to play the map just as well or better than you, and will generally (hopefully) be better than you.
Classes with a lot of mobility are going to generally rock on this map. They can better abuse the terrain, or take advantage of the amount of cover. Scouts and soldiers are going to be the most dangerous class, followed (situationally) by demomen, snipers, and heavies.
If you are trying to re-take the midpoint, your first step should be to know where every single enemy player is, and what class they play. This is essential, because it tells you which areas are more dangerous (for example, if they have a sniper next to the house walkway, you're going to want to treat the left and center pathways with more caution) and what you need to expect. As you push, it is essential to watch for any enemies to slip around your line for a flank. On a map like viaduct, it takes only about five seconds for a scout to move from one side of the map to the other horizontally, so a flanking scout with good aim can absolutely ruin your chances of successfully pushing out.
Once you've made it to the mid point, the goal is more about containment of the enemy than eradication. If you don't have an enormous advantage of numbers, ubers, or skill, pushing past the houses on the enemy's side of the map leaves you incredibly vulnerable to flanks, especially because the opposing team will have faster respawns, assuming you just capped the point. Once you have a significant foothold on the mid point, try to secure as many chokes as you can. For example, have the demoman set traps and spam grenades down the stairs and under bridge next to the stairs, and send a scout or a soldier to watch over the cliff area. Again, the goal here is not so much to be able to kill the enemy, just to be able to hold them back as long as possible. After all, even if your team is getting outfragged, having a positional advantage will allow you to hold the middle point for longer, giving you the win.
If you opt to use kritzkrieg instead of uber on this map, you generally will want a scout to stay closer to the medic, to give additional protection during pushes. While the pocket soldier is usually more than enough firepower, it is so easy to get flanked on viaduct that the medic will usually get killed on pushes. Keeping a scout nearby ensures that you have another set of eyes looking for threats, and, if the scout is no longer needed, he can quickly dash off to the front lines.
Running sniper on viaduct is extremely tempting, given the amazing lines of sight that the buildings give you. However, if you cannot kill at least the other team's sniper, you are a detriment to your team. Oftentimes, the other team's sniper won't be very good either, hitting maybe 10% of his shots (we assume he's terrible!). However, his presence, and that 10% chance, means that you either can never move in his line of sight, or will have to pop uber extremely early, just to be safe. Successful countersniping gives your medic a higher chance of staying alive, and also ensures that the enemy team will constantly be distracted, looking out for your sniper.
32Man Pubbing Format
Now, 32 man public servers don't have a reputation for being incredibly challenging, but on a small map like viaduct, they can be tough. The two biggest problems by far are snipers and spam. Demomen can shut off areas of the map just by sending volleys of grenades in their general direction, and snipers will bodyshot anyone and everyone, slowly chipping down enemy health. I have two pieces of advice for these matters:
To beat demoman spam, you're just going to have to be more aggressive, and more effective at killing. Demomen will easily defeat you at medium to long range, because they can damage you without any sort of line of sight. However, if you rocket jump towards them as a soldier, for example, you can dish out damage extremely quickly, and take away most of their ability to land direct hits.
One can usually assume that snipers in public servers playing koth_viaduct will congregate in two main areas: the far left and far right sections of the map. The cliff/battlement area, popular for its immense amount of cover and lines of sight across the entire map, definitely offers the best vantage point. However, it leaves snipers open to the second location: the stairs. Snipers on the stairs have a bit of counterintuitive cover, as the lower ground in the stairs region prevents many players from actually seeing them. However, it can only cover the mid point and opposing cliff area, and should really only be used to countersnipe, or take out an annoying player on the cliffs. If you trust your sniping aim immensely, but are afraid of moving close to combat, definitely position yourself on the cliff. However, you are a ripe target for any player, as flanking you is exceptionally easy, and an almost guaranteed kill.
For a bigger challenge, but a better chance of more kills and less deaths, you should try to snipe from the two middle entrances to the point, under both bridges. You'll be surrounded by your teammates in most cases, and will be an unexpected threat to snipers in the aforementioned locations. Additionally, you'll have great lines of sight to most of the enemy's side of the map. Scouts are an enormous threat, however, and you should be fairly good at dodging demo spam to survive (hint: spend less time zoomed in, as this makes you a sitting duck).
The best classes to play in a public koth_viaduct map are usually medic, demoman, sniper, or soldier. Scouts will tend to accumulate too much spam damage to be effective, and the sheer number of players on each team limits their ability to flank. The same can be said for pyros. Heavies are huge targets on viaduct, because they can do massive amounts of damage on the small map. If the enemy team has no snipers, then playing heavy becomes a walk in the park, provided your team has enough firepower to combat the enemy. Engineers, when fully situated, can be an incredibly annoying force on koth_viaduct. Placing a sentry in the bush next to the stair-side bridge will allow you to cover the point, and is almost impossible to see. However, it will quickly get spammed down from covered locations, so be prepared to rebuild.
I have seen some spies do incredibly well on 32man koth_viaduct, but in my experience, this is exceptionally difficult. I am currently interviewing a few players who are consistently successful at playing spy on challenging maps, so expect more on that later!
Hope this helps, and as always, feel free to send me questions about anything you'd like covered!