Complex sentence: The dome originally had a nude statue of Mercury on the top, which was removed in the 1920s, either because it was unstable in poor weather or was viewed as sinful by local residents.
Split into simpler sentences: The dome originally had a naked statue of Mercury on the top of it before it was removed in the 1920s. The reasons given for removal were either because it was either unstable in poor weather or viewed as sinful by local residents.