Nexus of the Worlds. The Askinar Towers, #1

Sara's tenth birthday was concluded by a visit from the family friend known only as The Professor. He gives Sara a special key that starts her on a great adventure! Along with her sister, Erika, she gains access to a world dominated by four towers. Their goal is to find a way back home and that means finding the Fifth Tower, which is considered a legend. Along the way they join up with a detective from 1935 New York, a female secret agent from 1969 Washington D.
C., and a King's Champion. Together they encounter obstacles that delay their travels. But someone else is ever present watching the progress of the girls and their friends. A black robed figure trails behind with an agenda of some sort. Does it want the key or the girls?