Complex sentence: They decided to build it in ten Boom's bedroom; as it was in the highest part of the house, people trying to hide would have the most time to avoid detection (as a search would start on the ground floor).
Split into simpler sentences: They decided to build it in ten Boom's bedroom, as it was in the highest part of the house. This would give people trying to hide the most time to avoid detection (as a search would start on the ground floor).