This map may have had the most changes in strategy not only due to the dispute over ladder-blocking, but the map changing all together. The map was updated around summer 2011. Since the update concert provided a better versus map, survival has had to adapt to new spawn points and old strategies crumbling. Although common infected are easy to kill and you'd rather focus on something like a special infected or tank(s), they serve an important role in the game. The combination of an endless horde, special infected, and tanks balance the games difficulty so removing one of these three would make a simple game. When all four play on the sniper tower it’s impossible for any common to play their role by blocking their climb. Not to mention the blocking of anything else climbing up. The description below is how we played concert to achieve the world record without blocking anything.

One player was on the sniper spawn scaffolding and did not block any ladders. The three other players played near the 2 medkits at the top of the bleachers. We played this way for the majority of the round but the first 9 mins we defended the stage in hopes of a better special infected count. Common infected may be funneled in by the guard rails near those 2 medkits so that one player is free of horde to focus on the tanks/SI while the other 2 focus on not letting common in. 

(Front is facing the stage from the top of the bleachers)

• 1 Player on the scaffolding spamming the sniper, shooting in all directions (A).
• 1 Player is on the right funnel guard rail focusing the right and protecting the scaffolding player (also referred to as the back) (B).
• 1 Player is on the 2 medkits shooting in all directions (middle) (C).
• 1 Player is on the left funnel guard rail focusing the left (front) (D).

• The player guarding the back controls most of the gascans \& fireworks (which we store on top of the vending machines), this player also warns the tower player if anything is climbing up behind them or shoots anything climbing up behind them when they have time as well.
• The middle player watches every side and helps out the front-side player a lot, as well as shooting the tanks coming from all directions.
• The front player gets a lot of horde coming in, and lots of tanks come to the front as well. The tanks should mostly be dead by the time they reach the 3 players because the front and tower players should be chipping them. The front player also lays out gascans and fireworks on their side. 
• All players are using military sniper rifles, when it's clear the 3 players on the bleachers grab ammo from the sniper spawn and quickly get back to their spots.
• The scaffolding player must be a stronger sniper with endurance to spam at the maximum speed the entire round.
• The horde must be funneled in and prevented from slipping past for the ground players.
• The ground players primarily used a military sniper to prevent running to the far ammo pile at the stage.
• Keep the supplies near the vending machine with the gas on top of the vending machines (seen in figure). The guns need to be where you funnel in the common.
• When a player is down, a throwable must be used to pick that player up safely.
• Gascans are a great way to kill a horde or light an unchipped tank.
• Since the middle ground player is mostly free of horde that player must be focusing the tanks and SI while helping with the horde when possible.
• If the tank climbs up the scaffolding warn that player to escape before it's too late.
• Don't wait until you're out of ammo to get a new sniper, grab one when you're on your last clip or after a double tank.
• Calling the direction in which the tank is coming from will give you the best results, so construct your own front and back before the round starts so no one is confused.
• If 2 or more players die it's best to run around the perimeter of the map to stretch a couple mins.
• Lots of spitters target the player on the scaffolding, so when there's spit just jump on the railing until the spit is gone.

by GuyGuy