Season of the Witch. Called Through Time: Highlander Brides, #4

The king is furious when word reaches him Eilean Donnán was conquered by Vikings. Adamant the stronghold is returned to Scotland at once, he dispatches the ruthless Highlander warlord Cainnech MacKenzie to lay siege. If Cainnech can defeat the Nords and recapture the island by year's end, the land, castle, and titles that go with it will all belong to him. Cainnech expects to make war against a Viking jarl, but is shocked to discover the current master of Eilean Donnán is no man at all.
The Viking is a woman... and the woman is rumored to be a witch. Lucia Ingegärd is an American of Italian and Nordic descent. Or at least she was an American until she was transported to a time long before her country existed. Desperate to return to the future before Christmas, Lucia puts her engineer's mind to the singular focus of going home. Her work is stalled when a huge, fierce-looking warrior surrounds her temporary home and gives her an ultimatum: surrender herself or she'll be shown no mercy.