Complex sentence: In the next 100 years the main Dutch defence line would be the new water line which was further extended and modernised in the 19th century with forts containing round gun towers reminiscent of Martello towers.
Split into simpler sentences: In the next 100 years the main Dutch defence line would be the new Water Line. It was further extended and modernised in the 19th century, with forts containing round gun towers reminiscent of Martello towers.