On Oct. 14, 1645, at the height of the English Civil War between Charles I and his defiant Parliament, Gen. Oliver Cromwell unleashed a brigade of the New Model Army against the most notorious remaining royalist outpost. For more than two years, Basing House—a sprawling estate in Hampshire, 55 miles southwest of London—had been held for the king despite repeated efforts to subdue its garrison. Now, at last, the strongpoint was stormed, and all inside were slaughtered or captured.

In “The Siege of Loyalty House,” Jessie Childs tells the compelling story of a place that acquired a mystique far beyond its strategic significance, mounting a staunch resistance justifying the sobriquet recalled in her title. Underpinned by meticulous research, this finely crafted narrative unfolds in evocative and often poetic language, transporting readers back to a “terrifying, electrifying time” and breathing fresh life into the men and women who endured it.

The Siege of Loyalty House

By Jessie Childs

(Pegasus, 318 pages, $28.95)

Ms. Childs, a historian whose previous works have focused on the earlier Tudor period, shows how the hardships of enforced confinement revealed the best and worst of Basing’s defenders. It was a “garrison of all the talents,” she writes, and included individuals whose backgrounds as artists, scientists and merchants open vistas into a tumultuous age. The fight for Basing House becomes a prism to view the English Revolution, a much-debated episode encompassing the execution of Charles I in 1649 and the monarchy’s replacement by a decade-long republic. For bewildered witnesses, it truly was a world “turned upside down.”

Highlighting key flashpoints, Ms. Childs traces the gradual polarization of loyalties and the inexorable slide into war. The stubborn, duplicitous Charles Stuart believed in his divine right to rule as he pleased, but Parliament refused to bend to his will, suspecting him of seeking to revive the elaborate “papist” rituals of the pre-Reformation Catholic Church. As tensions escalated in London, the king’s swaggering Cavalier supporters confronted gatherings of Roundheads (a mocking reference to the city’s short-haired apprentices who favored Parliament).

Ms. Childs charts the Civil War’s unspooling tragedy with insight and compassion. Shocked by the opening clash at Edgehill, in October 1642, Parliament’s commander in chief, Robert Devereux, Earl of Essex, was unable to compose the customary postbattle report. After experiencing the carnage, the earl’s “mind and body shut down.”

Like other fratricidal conflicts, the struggle pitted former comrades against each other. In younger days, the Roundhead general Sir William Waller and his opponent, Sir Ralph Hopton, had shared the dangers of wartorn Bohemia (today’s Czech Republic). In December 1643, when Hopton wrote Waller requesting the body of a slain subordinate, he earnestly desired that “God give a sudden stop to this issue of English blood” before signing off as “your faithful friend.”

Although religious wrangling contributed to the war, for many, when it came to picking sides, conscience trumped faith. The owner of Basing House, John Paulet, 5th Marquess of Winchester, was both a devout Catholic and an ardent royalist whose family motto, “Aimez Loyauté” (“Love Loyalty”), gave his home its nickname. By contrast, some of the stronghold’s doughtiest defenders, including its military governor, Marmaduke Rawdon, were Protestants who had anguished over their allegiance.

Located on the main road to the west country, Basing House had long been a convenient stop for traveling monarchs. It featured two brick-built Tudor mansions: The Old House was sited within the substantial earthworks of a Norman castle; the more elaborate New House was constructed alongside it. Erected as a peaceful residence, Basing House was nonetheless defensible against all save the heaviest artillery. As the siege intensified, its walls were bolstered by up-to-date earth bastions, likely designed by a celebrated occupant, the architect Inigo Jones.

The divisions that sparked the English Civil War, Ms. Childs tells us, prompted a proliferation of “newsbooks” and partisan pamphlets—the birth, she says, of “popular journalism in England.” Another of Basing’s notable defenders was Thomas Fuller, whose biographical collection, titled “History of the Worthies of England,” aimed to preserve a kernel of truth amid all the circulating lies.

Among the most fascinating—and appealing—members of the garrison was Thomas Johnson, a London apothecary who rambled far and wide to find and identify “every indigenous plant in England and Wales” so that he could catalog and analyze the ingredients that went into the medicines he prescribed. Johnson proved himself a natural soldier, his dedication as a herbalist matched by bravery in combat.

Ms. Childs possesses the knack of encapsulating action in a sentence or two of lively prose and telling imagery. Roundhead cavalry routed over a steep hill during the 1643 battle of Roundway Down become a “broth of metal, bone, leather and flesh,” while mortars deployed to lob explosive shells high over the walls of Basing House are likened to “squat iron toads” that “croaked deadly fireworks.”

At the restoration of Charles II in 1660, the Act of Oblivion sought to erase memories of recent strife in hopes of future harmony. One group was exempted from amnesty: The surviving “regicides” who had urged the death of the king’s father faced the ghastly punishment for treason. Mercifully, Ms. Childs spares us the gruesome details, yet her description of the firebrand preacher Hugh Peter awaiting death, determined to master his fear despite the jeers of the gore-smeared executioner, is no less chilling.

Cromwell’s bloody conquest left “Loyalty House” a scorched shell. Even so, its ditches and foundations survive. Early last century, excavations exposed graffiti left by Roundhead prisoners. The sketches were swiftly obliterated by the melting snow of an unexpected storm, yet Ms. Childs’s vivid account will ensure that the drama that inspired them won’t be forgotten.

Mr. Brumwell’s books include “Turncoat: Benedict Arnold and the Crisis of American Liberty.”