The film is based on a novel by William Eastlake published in 1965. Eastlake enlisted in the US Army in 1942. He served in the Infantry for four and a half years, and was wounded while leading a platoon during the Battle of the Bulge.
The film opens with long, beautiful shots of ancient European art and sculptures being blown to pieces amidst the sounds of war and dissonant screams; a lone narrator begins his tale of "eight American soldiers" as the scene abruptly flashes back to a few weeks beforehand. Prior to the Battle of the Bulge, a ragtag squad of American soldiers (strongly implied to be some sort of replacement outfit), led by one-eyed Major Falconer (Burt Lancaster) and including Sgt. Rossi (Peter Falk), art expert Captain Beckman (Patrick O'Neal), and the highly intelligent narrator and sole African-American, Pvt. Allistair Benjamin (Al Freeman, Jr.), takes shelter in an ancient Belgian castle, the Maldorais, containing many priceless and irreplaceable art treasures. Although Falconer begins an affair with the young and beautiful Countess, he is surprised to find the Count (Jean-Pierre Aumont) encouraging him; in fact, the impotent nobleman hopes the Major will impregnate the Countess so that his line may continue. Meanwhile, Beckman begins to butt heads with Falconer over both the value of the art (in the context of either saving or destroying it in the event of a German assault) as well as Beckman's own unrequited attraction to the Countess, who seems to symbolize the beauty and majesty of the European art he studied before the war. The enlisted men seek their own pleasures in the brothel of the nearby town, the psychedelic "Reine Rouge" (Red Queen) run by a mystical madam, whilst Beckman marvels at the castle's artworks, many of which are stored beneath the castle for safekeeping. Sgt. Rossi, a baker before the war, falls in love with a baker's widow and decides to go AWOL, resuming his pre-war life; another soldier falls in love with a Volkswagen Beetle; his affection for the foreign vehicle borders on paraphilia and becomes a long running and anachronistic gag throughout the entire movie.
The film from this point on begins to reach a surreal climax, as the soldiers' days of leisure and peace threaten to undermine the very reality of the war itself. A recurring theme throughout their escapades is the very idea of eternal recurrence itself, as one soldier drunkenly ponders out loud if maybe he's "been here before". Although the men are eager to sit out the war that they feel will soon end, the experienced Major Falconer predicts that Germans will attack the thin American positions in the Ardennes and that the castle is a strategic point in the Germans advance towards the crossroads of Bastogne. The Major's theories are confirmed when he sees German star shell signals and successfully ambushes a German reconnaissance patrol led by a German officer who was once billeted in the castle and was a lover of the Countess as well.
Captain Beckman and the Count are horrified that the Major will not abandon the castle, a decision that will surely lead to its destruction; Falconer, however, is adamant that to give the Germans one thing means that they'll just end up "taking everything" later on (see appeasement). Falconer prepares defensive positions around the castle and sends his unit into town. The Germans are initially taken by surprise, as Falconer directs the local sex workers at the "Reine Rouge" to draw them into a trap with Molotov cocktails; however, the defenders soon find themselves outnumbered and outgunned (although two GIs manage to steal and repurpose a working German tank, which they jokingly claim is "better than ours"). Seeing no other choice but to retreat to the safety of the castle, Falconer attempts to rally shell shocked American troops retreating from the Ardennes into the Maldorais, forcing (at gunpoint) a band of zealous, hymn-singing conscientious objectors, led by Lt. Billy Byron Bix (Bruce Dern), to lead the dazed survivors in a bizarre Pied Piper-esque procession; symbolically, they are all mostly killed by an exploding shell, all except for Falconer, who stoically returns to the castle for his last stand astride a pale white horse. He returns to find that the Count has run over to the German lines; Beckman thinks he has a scheme to betray them and let the Germans seize the castle by using the underground storage tunnels to gain access; however, it is soon revealed that the Count was really only trying to buy as much time for the Americans as possible so that they could make it to the castle and strengthen their defenses. As soon as his ruse is discovered, he is gunned down trying to run from the Germans. Falconer and Beckman put aside their personal and ideological differences and grimly prepare for the oncoming assault with a .50 caliber machine gun pointed across the castle grounds.
At the conclusion of the film, everyone defending the castle—with the exception of Pvt. Benjamin and the pregnant Countess, who escape to safety using the art storage tunnels, following the last, direct orders of Maj. Falconer—is killed by waves upon waves of besieging Germans in an absurd battle scene featuring the enemy storming the gates using the ladder-carrying fire trucks, and much of the castle (along with its art treasures) is eventually obliterated by artillery, incendiaries and other modern-day weapons. Falconer, the last defender left alive, begins to think of all of the people whom he has killed or have died because of his actions as well as the Countess as he guns down the rapidly approaching swarms of German soldiers, implying that he did indeed feel guilty about their deaths and that he had indeed loved the Countess much more than he had ever let on; a shell finally lands on top of his position and explodes as the screen goes white. The film ends where it began, echoing the theme of eternal recurrence, with more long shots of the undemolished Maldorais as it once stood as well as a voice-over of Benjamin's narration from the very beginning before the scene fades to black.Burt Lancaster as Major Abraham Falconer
Patrick O'Neal as Captain Lionel Beckman
Jean-Pierre Aumont as The Count of Maldorais
Peter Falk as Sergeant Rossi
Astrid Heeren as Therese
Scott Wilson as Corporal Clearboy
Tony Bill as Lieutenant Amberjack
Al Freeman Jr. as Private Allistair Piersall Benjamin
James Patterson as Elk
Bruce Dern as Lieutenant Billy Byron Bix
Michael Conrad as Sergeant DeVaca
Caterina Boratto as Red Queen
Olga Bisera as the Baker's Wife
Beckman: "Hey Benjamin, now there's a title for your book... 'Castle Keep'..." Benjamin: "...Not bad..."
Rossi [confronting the deserters]: You're waking everybody up, go back to your outfits! Deserter: We have no outfit. We don't believe in fighting. Rossi: Who does?
"Europe's dying." "No Beckman, shes's dead. That's why we're here. Don't you read the newspapers?' - Major Falconer
The film was shot in Novi Sad, Serbia. Sydney Pollack recalled that Burt Lancaster first wished him to direct the film in 1966 and that the castle which was made of styrofoam was inspired by Walt Disney and dreams.