The House at the Crossroads — Thomas T. Thomas

Thrill-seekers from the late eighth millennium, Anja Varden and Rolf Pohl, sign up with a secretive research organization to carry the seed of a time-travel portal across medieval Europe from Rome to London. In doing so, they will establish a new node in the organization's planned network of access points into humanity's ancient history. But before they can set out, the couple must first change their identities and their priorities, learn a whole new way of life based on primitive skills from a long-gone era, and sign a service contract with no escape clause. It's do or die: either they cross the continent, set the seed, and return by the quantum pathways the device will open -- or perish.

At the same time, but starting in the well-ordered society of the eleventh millennium, young misfit Coel Rydin commits an act of personal violence. He is brought before a citizen tribunal that condemns him to enlist with the mysterious Troupe des Jongleurs. These newly founded guardians of traveling in time can make good use of his aggressive tendencies. The Troupe pairs him with a Silicate intelligence, product of an earlier time-travel accident, who introduces himself as Cinquemain. The machine mind will act as his pilot and guide as they venture back into pre-history. Their first assignment is to search out and destroy the ancient Roman node established by a band of time travelers they know only as “the Builders.”

Rydin and Cinquemain locate the node and manage to destroy it. But on returning in triumph to the eleventh millennium, they learn that a second node has already sprung up in ancient London. Finding and stopping this node at its inception point requires them to dance to the music of the celestial spheres. In hunting back and forth across time to locate this new portal, the pair must solve mysteries of time and space, and expose themselves to new dangers in a history the Jongleurs barely understand. And as for Anja and Rolf, they must fight enemies they know only as clouds, shadows, and fairy-tale demons.

Prequel to the time-travel novel The Children of Possibility, this book carries on a wide-ranging battle that stretches from Nero's Great Fire in the Rome of 64 AD, through the Norman Conquest, the Crusades, the Black Death, the Wars of the Roses, the bloody reign of Mary Tudor, and the Great Fire of London in 1666 AD. Along the way, the Jongleurs attempt to destroy the node more than once. But time is a slippery commodity and has a way of curving back on itself.