Complex sentence: Christian Frederik Hansen, until then master builder in Altona, was called upon to resurrect the palace and construction started in 1803 but progressed only slowly due to the dire times with the Napoleonic Wars and the national bankruptcy in 1813.
Split into simpler sentences: Christian Frederik Hansen, until then master builder in Altona, was called upon to resurrect the palace. Construction started in 1803 but was slow as a result of the difficult times compounded by the Napoleonic Wars and the national bankruptcy in 1813.