You can have readied actions without a door. However, there is a significant difference: both sides are not only aware of each other but are capable of interacting. This is a much rarer example because there are actually drawbacks to taking readied actions and whoever readies is passing up the opportunity for a full round action that catches their foes flatfooted and is actually reducing their initiative score in a significant way.
The shoot-out at the O.K. Corall scenario is instructive. Let's take seven characters: Doc Holiday, Virgil Earp, Morgan Earp, Wyatt Earp, Ike Clanton, McLaury, and Joe.
Init:
Doc
Virgil
Ike
Morgan
McLaury
Joe
Wyatt
Round 1:
All ready actions. Nothing happens: tense standoff.
Round 2:
All ready actions until it gets down to Joe. Joe's nerves are frayed. He decides to try a pick pockets roll to draw his gun without being noticed so he can get his shot in before rather than after the readied actions. (The DM has to rule on this since it's not accounted for in the core rules--let's say that he does have the Quicker than the Eye feat so the DM lets him bluff as an MEA vs. everyone's spot.) He beats everyone but Doc.
Doc sees Joe as Joe readies his gun and blasts him with the shotgun.
This triggers Ike's readied action to shoot Virgil. (And McLaury's actions too but that has to come later)
Ike triggers Morgan and Virgil's actions to shoot.
McLaury's action goes next in the initiative order.
Then Wyatt's action.
Finally, Joe gets his shot off (too bad Doc beat him on the spot roll). Joe takes a full round action and rapid shots, getting three shots off because he's a 6th level gunfighter with the rapid shot feat and a +6 BAB.
In this scenario, the Earps actually would have actually got more attacks off before the Clantons if they'd not bothered readying.
Ordinary initiative: (3 attacks each from BAB and rapid shot).
Doc: 3 shots
Virgil: 3 shots
Ike: 3 shots
Morgan: 3 shots
McLaury: 3 shots
Joe: 3 shots
Wyatt: 3 shots
Earps: 12 shots. Clantons 9 shots.
With readied actions:
Doc: 1 shot
Virgil: 1 shot
Ike: 1 shot
Morgan: 1 shot
McLaury: 1 shot
Joe: 3 shots
Wyatt: 1 shot
Earps: 4 shots. Clantons: 5 shots
Had the situation played out differently and McLaury started the shooting, it would have been:
Doc: 1 shot
Virgil: 1 shot
Ike: 1 shot
Morgan: 1 shot
McLaury: 3 shots
Joe: 3 shots
Wyatt: 3 shots
Earps: 6 shots Clantons: 8 shots
Had McLaury known that he would likely be the Earps' target and started acting by fighting defensively with expertise to full, it's likely that none of the three readied shots would have landed, putting the Clantons in an even better position.
Also note, that the initiative unfolds in roughly the same order despite readying. Neither the Earps nor the Clantons gain a significant boost in initiative from readying.
None of these cases are true in the "both sides aware but unable to interact scenario" which is much more common.
Murrdox said:
Heck, you can have a scenario with readied actions WITHOUT a door, and it'd still be confusing as all hell. Unfortunately, that's just what you'd have to DO in a situation where both sides are aware of each other, but neither side wants to go first... consider the shoot-out at the O.K. Corale:
Doc Holiday: I ready my shotgun to fire at the first cowboy that draws a pistol
Virgil Earp: I ready both pistols to fire at McLaury
Morgan Earp: I ready my rifle to fire at Clanton
and so on etc... and both sides continue to "ready" actions until someone does something.