What is the state of Elsinore in act 1, scene 1 to scene 5? Why are there so many guards?

In the opening scenes of Hamlet, Elsinore—at least on the outside—is in a state of high tension. The battlements are strewn with soldiers, heavily guarded in expectation of an imminent armed invasion from across the water. The Norwegian prince, Fortinbras, is a headstrong young man who's determined to win back all the territories lost to Denmark, and he's prepared to do this by force of arms. So the Danes are on high alert, prepared to defend their country from the expected Norwegian invasion.
Yet in Elsinore's great drinking hall, the atmosphere is completely different. King Claudius and his court are having another riotous drinking party, a custom that in Hamlet's disapproving words should be more honored in the breach than the observance. In other words, he doesn't think that it's appropriate to throw such a party with Denmark apparently on the brink of a foreign invasion.
To Hamlet, this is further evidence of the country's moral decline since Claudius usurped the throne. There's something "rotten in the state of Denmark," something nasty that corrupts the whole of public life, something that will sap the Danes' vitality, making it easier for the Norwegians to invade and present themselves as noble liberators.