Complex sentence: Trade unions were legal, but due to them swearing an oath of allegiance, they were arrested and tried in the Shire Hall in Dorchester which is preserved as it was at the time.
Split into simpler sentences: Trade unions were legal, but due to them swearing an oath of allegiance, they were arrested and tried in the Shire Hall in Dorchester. This building still remains and is preserved as it was at the time.