Complex sentence: Pula's fortresses differ from the original Linz fortress in that older fortresses built between the years 1851-1855 are smaller and weaker than the ones built ten years later, like Fort Bourguignon.
Split into simpler sentences: Pula's fortresses differ from the original Linz fortress. Older fortresses built between the years 1851-1855, are smaller and weaker than the ones built ten years later, like Fort Bourguignon.