During the Tang dynasty , a navy town, Tuen Mun Tsan  was established in Nantou, which lies across Deep Bay. Tuen Mun and the rest of Hong Kong were under its protection. A major clan, To , brought the name Tuen Mun to the area.  They migrated from Jiangxi on the Chinese mainland and established a village Tuen Mun Tsuen  late in the Yuan dynasty .  As more and more villages were established, the village was renamed Tuen Mun Tai Tsuen , which means "large village" in Chinese.  As yet more villages were established, a market town of Tuen Mun Hui   was established. This town lies where present-day Tuen Mun Kau Hui is situated. Tuen Mun remained an important town of coastal defence until the start of British rule in 1898. When the British took over the New Territories from the Qing government in this year, the area was renamed Castle Peak, and Tuen Mun Hui to Castle Peak Market  or Tsing Shan Hui. The name Tuen Mun, however, continued to be used by those living in the area. In 1965, "Castle Peak New Town" was planned. It was later renamed Tuen Mun New Town and constructed from 1970 onwards with many buildings on the reclamed land of the former Castle Peak Bay. The name was officially changed back to Tuen Mun in 1972. The first public housing estate built in the town was Castle Peak Estate, opened 1971.
Question: How many years after Castle Peak New Town was planned did Castle Peak Estate open?
Answer: 6