Don't let anyone fool you. A match of Well does not start when the gates open. What makes this map unique is that both teams can focus on preparation.
If you are an Engineer, it is of utmost importance that you set up the Teleporter Entrance the very first second you can. This map has a lot of travel time from spawn to center, so herding along reinforcements is beyond helpful. A suggested spot is beneath the central point, in the sideroom with entrance to the water.
If you want to kill somebody, though, you should take a look at those railcars on the side of the big building. You can shoot under them, to the side of them, heck, you can even jump over them if you are so inclined.
But the class who really excels before the clock runs down is the Sniper. The enemy dumbos will probably be peeking right through the gates to see the middle point (for reasons unknown,) leaving you with an easy instakill, until they wise up. Still, if they remain standing in one place, you'll be privy to some memorable feet kills.
Once the gates finally open, it is the job of every person on the team to rush in and try to take the center point, obviously. A Medic should deploy their Ubercharge as soon as it becomes available because the risk of death is so high in the first seconds of this match.
If things are looking good, send a disguised Spy out to the Transitions Point to attempt a quick capture. If things are looking bad, the survivors should busy themselves with distracting the other team and slowing their progress to the next point. The Scout, for example, can effectively stop an entire team from moving forward by making a large circle in the center building, continually grabbing the health packs.
No matter how ugly it may be, though, don't stoop down to the level of taking enemy Teleporters offline by sapping the Entrances. There is something about that action that screams that you are a skill-less husk of a player.
Once your team has formed a presence on the capture point, a Demoman should visibly place sticky bombs in the two ground entrances. This scare tactic can temporarily keep the enemy team out, even if the Demoman isn't watching his trap.
To summarize, keep the other team as distracted as possible to stop them from moving forward, if that be to your Transition Point or another wave of enemies to the Center Point.
If you are defending this point, the best thing you can do for yourself is to keep checking into the Resupply Room (the one that doubles as a Spawn Room if you capture the Central Point.) No matter how intense the firefight is on the Transition Point, it is always a short retreat to full health and an instant reload.
Regardless, on a team of eight, only two or three people should be guarding this point. The rest of the players should be high-tailing it out to the Central Point, because you can't win the match without capturing it. If the whole team plays defense, it is setting itself up for stalemate (at best) or defeat (pretty likely.)
A Heavy, Pyro, and Demoman can lock down this point by themselves if they make regular trips to the Resupply. An Engineer isn't so helpful here because the geometry prevents a single Sentry from blocking a majority of the pathways. Instead, the team Engineer should go offensive and try to make some real progress.
As it takes so long to get up to the Sniper Deck, I would discourage any Snipers from trying to defend from that location. Like the Engineer, they are more useful being offensive.
Medics can make a massive impact when trying to capture this point, so if you are defending and the other team pops an Uber, run back to the nearby Resupply Room and wait it out.
Attacking this point is a whole different beast. Once inside the building, sight lines are very short and the list of movement options is short. Thus, a forward press of Soldiers and Scouts is unlikely to succeed as quickly as a barrage of Heavies and Pyros.
A Spy will be right at home here with many corners and dark places to hide in. Additionally, they have easy prey waiting in the Sniper deck. If your team begins to capture this point, it is your duty to get to the Final Point as fast as you can to attempt a quick capture.
If your team is pressed back this far very suddenly in a match, press forward right away. The opposing team is unlikely to have much offensive backbone, instead having had a quick cap on the Transition Point by a Scout or a Spy. A Heavy or a Pyro can do an excellent job of guarding the Final Point from a stray Scout or Spy while the rest of the team captures the Transition Point.
Many teams, instead of doing this, turtle up defensively and wait for the match to end in a stalemate or their defeat. Unless the entire enemy team is on your doorstep, only three or four players should defend this point while the rest of the players move forward.
The worst habit you can get into is "preparing" for the siege as an Engineer at the beginning of the match. You know who you are: the ones building Dispensers and slowly making their way up to the rafters. Sure, it might save you in the end from total failure, but it certainly isn't helping your team win.
If you are attacking as an Engineer, try to set up camp on the third floor. This valuable property will allow your team to reap the benefits of height advantage without climbing three flights of stairs, and is also difficult for the other team to recapture.
In the event that your team captures Transition Point, you should regroup than move forward together. In the meantime, a Scout or two can pester the enemies to buy you more time.
If you manage to build up an Ubercharge, be sure to use it wisely. Wait as long as you can to deploy it, and hope that someone doesn't get lucky and Crit the Medic. If you find yourselves alone on the Final Point and Ubered, the Medic should switch weapons so that he and his patient can capture the point before more enemies come.
If it is apparent that the other team is turtling, the team should try to make an attack from the balconies surrounding the Point. The vertical difference allows Soldiers to bombard the defense and Snipers to make a massive impact.
If your team is communicating, a good strategy is to snipe off the Engineers and Pyros while friendly Spies sap the Buildings. After weakening the team this heavily, your victory is imminent.
Well is one of my favorite maps if it is played correctly. Teams will often go back-and-forth over control of the Center Point, showing how much deeper the strategies and teamwork are on a 5-point symmetrical than an Attack/Defend like Dustbowl.
Suggested Improvements: The good news is that the issues I bring up have mostly been fixed on patches for the PC edition and are coming our way. The biggest problem is the ridiculous travel time from the ground floor to upper floors. An extra staircase will be added, but I also think that one of the windows on the Sniper Deck should be "busted out" for Soldiers and Demomen to jump through. Otherwise, I think time before Sudden Death should be a minute or two longer. Some epic charges have been ruined by that timer.