"Neuchatel contacted me last May but I told them I was under contract with Zamalek," the Swiss tactician told Arcinfo.com.
Decastel was named as the Cairo giants' boss in January in a bid to turn around their fortunes after a dismal run in the Egyptian Premier League.
Zamalek, who faced the unthinkable prospect of being relegated to the second division earlier in the season, put in some fine displays under the former Etoile Sahel boss.
Decastel promoted several youngsters to the senior team as he sought to rely on fresh players. His work bore fruit as the White Knights finished sixth in the league.
"I first had to restore order and discipline in the team," the 53-year-old added.
"The pressure is obviously enormous but the challenge I face is beautiful. The club have won nothing for five years but everything is in place now to change that."
The 2008 Egyptian Cup was the only trophy Zamalek have won since clinching the league title in 2004.