Here is the full list of all 19 Hidden Through Time achievements. It takes around 4-5 hours to unlock all of the achievements on Windows.
Complete 50 online maps
Complete 25 online maps
Find All Objectives of all maps
Find All Western Objectives
Complete 5 online maps
Complete 10 online maps
Find an objective
Find 10 objectives
Find 25 objectives
Find 50 objectives
Find 100 Objectives
Find all objectives of a map
Find All Objectives of 5 maps
Find All Objectives of 10 maps
Find All Objectives of 15 maps
Find All Cavemen Objectives
Find All Medieval Objectives
Find All Egyptian Objectives
Complete an online map