Here is an interpretation of Fortinbrasse's battle to take Elsinore. This interpretation is based on the major themes in Hamlet, of "Putting On A Show," and "Madness."
Old Norway and Fortinbrasse were in league, in the raising of Fortinbrasse's army for use against Denmark, which Horatio mentioned in Scene 1. Unlike Claudius with Hamlet, Norway has been a good substitute father for Fortinbrasse, and has provided Fortinbrasse with support and guidance, and the wisdom of his years. Old Norway has no love for Denmark, since he was defeated by Hamlet Sr, and he would like revenge for his humiliation, just as Fortinbrasse wants revenge for his father's death. Old Norway has pretended to be feeble and unknowing, to deceive Denmark about his involvement with the Norwegian army recruitment.
When the Danish ambassadors arrive, Norway and Fortinbrasse put on a show for them. Norway pretends to rebuke Fortinbrasse, and Fortinbrasse pretends contrition. Norway also pretends to be an indulgent uncle who spoils his nephew. Norway says Fortinbrasse is a good-hearted boy who needs something to do. Norway suggests Fortinbrasse be allowed to have his army, but to fight against Poland, instead. Since Poland is a traditional enemy of the Danes, the Danish ambassadors easily agree to this.
Thus, we get the agreement for Fortinbrasse to fight the Poles. As a by-the-way, Norway mentions that it would be quite helpful for the struggle against the Poles if Fortinbrasse's army could travel through Denmark along the way. Pleased at how everything is going, and in the interests of peace between Norway and Denmark, the Danish ambassadors agree to this peaceful gesture. They all have a drink in celebration, with smiles and handshakes and toasts to each other, and then the ambassadors leave to return to Elsinore.
Fortinbrasse is impressed by his uncle's deceptive diplomacy, and listens carefully to the rest of what his crafty old uncle tells him.
The Danish ambassadors proudly return to Claudius with the agreement, and describe it in glowing terms. Claudius is very pleased at achieving a successful mission so quickly. He will not think very much about it, because he's so preoccupied with Hamlet. He signs the agreement, the necessary copies are returned to Norway, and the agreement is made known throughout the government of Denmark.
As we see in the play, Fortinbrasse lands his army at the docks of the town near Elsinore. Hamlet encounters the Captain who is going to the castle to get the official paperwork from Claudius which makes it alright for a foreign army to be in Denmark.
Left on his own, Fortinbrasse would have tried to attack the castle immediately, as soon as he had his army ashore. However, Norway has told him to be cautious, and to gather as much information as possible before making his move. Fortinbrasse heeds the good advice. He leaves a couple of agents in the town, and also manages to get agents into the castle, working as menial servants. This proves not too difficult, since castle security has been relaxed following Claudius's "successful" diplomatic mission.
Fortinbrasse takes his army a day's march to the west, and camps there. He intends to remain there, awaiting intelligence reports. If anybody asks why the army is stopped there, the cover story is that they have had to stop to await supplies. Anybody with military experience would easily believe it, that the men and the supplies are not in the same place at the same time.
Then, Fortinbrasse receives an extremely interesting item of news: a mob has stormed the castle! He is told about Laertes and his mob from the town. Fortinbrasse knows this is an opportunity too good to miss, and he orders his army to move out, toward Elsinore.
Fortinbrasse moves at a fast march, but still cautiously, in case the report turns out false or misleading. It sounds too good. The cover story now is that the army had to return to the town docks, to unload their own supplies.
The timing is such, that Fortinbrasse approaches Elsinore at the same time Hamlet is talking to Horatio before the fencing match. With the castle in sight, Fortinbrasse slows down, to a slow march, to appear non-aggressive while he peers at the castle. The castle shows no major damage. He continues to approach, carefully.
The fencing match is beginning now, and Fortinbrasse is fairly close with his army. He knows the soldiers in the castle can see him, but he has angled toward the town, and has his cover story ready in case of challenge. As he looks at the castle, it still looks normal. He's unsure whether to attack.
Inside Elsinore, Claudius drinks the toast to Hamlet, to start the fencing match. Notice the exact wording in the play. Claudius has ordered that "all" the cannons on the battlements be fired as a salute: "Let all the battlements their ordnance fire." The cannons will only be firing blanks to make noise. For a salute, the cannons are loaded with powder and wadding, but no shot.
Back outside, now. Suddenly, without warning, Fortinbrasse sees all the cannons at the castle fire! He thinks the Danes must have uncovered his scheme, and they're shooting at him to kill and disperse his army. His own personality takes over - recall that Horatio described Fortinbrasse as hot-headed, in the first scene of the play. Fortinbrasse has his own cannons, but he doesn't take time to deploy them and shoot back.
Fortinbrasse instantly decides that he has not come this far to retreat, and he cannot stay there to have his army cut to shreds by the Danish cannons. He orders the attack! "Charge!" screams Fortinbrasse, and spurs his horse toward the castle. His army immediately responds. They charge at Elsinore as fast as they can run, following Fortinbrasse.
The Danish soldiers defending Elsinore have noticed Fortinbrasse's army, and they quickly discuss it among themselves. They have no high leadership - the top officers are all at the fencing match! The Danes all know about the diplomatic agreement, and they don't understand why Fortinbrasse would appear to be attacking. They know they've only fired blanks with the cannons, so it doesn't occur to them he'd attack because of that.
Then a clever fellow suggests that since it can't really be an attack, it must be training. Fortinbrasse must be using Elsinore as a training ground, as he goes by, to help prepare his men for the attack on the objective in Poland. Everyone quickly agrees, well of course, it must be a training run, just like they, themselves, do every week or so. Periodically, their officers will divide the Danish soldiers into two groups, and one group will pretend to attack the castle while the other group defends it. The training is for physical exercise, and to give lessons on how to defend and attack. The Danes call out to their fellow soldiers to come to that side of the castle and watch Fortinbrasse train his men.
Inside, Hamlet has scored the first hit on Laertes, "a very palpable hit." The cannons fire a salute again, as Claudius ordered if Hamlet got the first or second hit.
Outside, it's "palpable" also, as Fortinbrasse's army charges Elsinore, when the cannons all fire again, right in front of them. With a new surge of fear and adrenalin, the army runs even faster as they scream their war cries.
Up on the battlements, the Danish soldiers talk to each other. They all agree that Fortinbrasse's army looks very impressive in making their practice charge. They say that if Fortinbrasse can charge his objective in Poland this well, the poor dumb Poles will never stand a chance. They talk about how sorry they feel for the poor Poles.
Fortinbrasse is very close now, and as he looks ahead he sees: open doorways! When Laertes's mob stormed the castle they broke the doors. Claudius said it, when Laertes came into the room where he was: "King. The doores are broke." The big doors have not yet been repaired.
When Fortinbrasse sees the open doorways, it looks much too good to be true, and he is suddenly fearful of a trap. But if it's a trap he's already in it, and there's nothing he can do but continue. He screams, "Charge! Storm the castle!" and leads his army in.
Now then, what do the Danish defenders do, as Fortinbrasse's army charges into the castle? The Danes think they've been watching a training exercise. In other words, they think they've been watching a show. What does anybody do when he sees a good show?
They applaud. Fortinbrasse enters Elsinore Castle to a standing ovation from his enemies.
As Fortinbrasse charges into Elsinore Castle, the Danish defenders clap and cheer. He's greeted by shouts of "Bravo!" and "Great charge!" and "Well done!"
When he sees the defenders cheering, Fortinbrasse is certain that's he's led his men into a horribly diabolical trap, and they're all going to die. Fortinbrasse still presses on. It's all he can do. He orders his army to take the battlements.
At this, the Danes shout approval. They call out, "yes, take the battlements!" and "very good!" The Danes, themselves, have done this a hundred times in training, and they know what an attacking group is supposed to do when it gets inside the castle. The Danes all quickly step aside, and encourage Fortinbrasse's soldiers, and they compliment Fortinbrasse for knowing what he's doing.
As Fortinbrasse's army takes the battlements and deploys inside the castle, the Danes simply move out of their way, while continuing to applaud and shout approval. Then there comes a pause.
The senior Danish officer on duty, a lieutenant, walks up to Fortinbrasse with his hand out for a handshake, and says, "Congratulations! That was the finest training run I have ever seen! You'll take those stupid Poles easily, if you can do that well when you get there!"
"Training" - Aha! Now, suddenly, Fortinbrasse knows what the Danes think, that it was only training. He shakes the officer's hand and says, "thank you!" Then he draws his sword and puts it to the Danish officer's throat, and says, "but actually, this was a real attack, and you're all prisoners." He calls out to his army to take the Danes' weapons. The Danes are so stunned they offer no resistance. The Danes begin asking how they can join Fortinbrasse's army, instead.
The battle is over. Fortinbrasse owns Elsinore Castle. There was not a single casualty on either side.
At the fencing match, four people died, and also Claudius's bodyguards that Horatio had to kill. Then there's R & G dead in England. But in the battle to take the whole castle, nobody was killed.
Fortinbrasse, the new ruler at Elsinore, is informed that some English ambassadors have arrived. He orders his cannons of Elsinore to be fired as a salute to them, and also just for fun. Now he gets to fire the cannons!
As mentioned at the top, this story of Fortinbrasse's victory at Elsinore has been written following the themes of "Putting On A Show" and "Madness" which are such prominent themes in Hamlet. Alternatively, Fortinbrasse's victory could be presented as a serious battle, which Fortinbrasse would win for several reasons. The cannons were only firing blanks, the doors were broken, and the Danish defenders were still bruised and tired from fighting Laertes's mob. A Fortinbrasse victory is inevitable, either way. But I think the "mad" version is more correct for Hamlet, with the defenders acting in a way that looks crazy, when they think it's only a show. There is also the significant point that the fencing match is not interrupted by anybody rushing in to announce that there's fighting outside. Therefore, it's in agreement with the onstage play events that no combat happened. I believe this "mad" version is more true to Hamlet, both thematically, and in agreement with the onstage events.
(Regained)