As with their previous two Yorkshire League Three outings, the game went to the wire and it was not until the 73rd minute that Castleford got ahead for the crucial time.
The game was a personal triumph for teenage wingman Lee Land, who was only allowed to start a rugby union career earlier in the season after disciplinary officials agreed to a transfer from rugby league after he had been suspended sine die from the 13-a-side game.
He showed good finishing skills to put his side ahead after 28 minutes, fly-half Steve Willett adding the conversion, and then produced an elusive piece of running to set up a walk-in try for centre Jason Dunwell that replacement Ian Mattison goaled with time running out.
In between, Phoenix shared the game and stayed in touch with a brace of penalties from centre Ben Millar.
They then went ahead after 66 minutes when replacement prop Alani Kaihau forced his way over from close range, leaving Castleford needing to dig deep into their reserves on a hot afternoon to rescue the game.
Land's solo burst finally undid the Phoenix defence and once in front, the West Yorkshire side took a grip of proceedings that Phoenix could not shake.
Doncaster's director of rugby Bill Farley said: "If we'd kept more control and made a bit better use of possession, it might have been different.
"I thought it was a cracking final and both teams can hold their heads high at what I thought was a good advert for Yorkshire Three rugby. We may have been slightly fitter on a testing day, but they held out and congratulations to them."
Castleford's matchwinner Land said: "It's been quite a year and I owe it all to Chris Willett, our director of rugby, who has helped me through the past two years to achieve everything I've wanted to achieve, including this award that I dedicate to him.
"But all credit to Doncaster because they didn't make it easy today. Every time we play them it's difficult."
Willett said: "Anxious doesn't quite cut it as the word to describe how I felt going into the last 10 minutes and trailing. We were very tired and that was a very hard game to play in at the end of a hard season where we've had to win all our games to get promotion. Every game has been a cup final.
Castleford: P Philpott; J Bramham, D Price, J Dunwell, L Land; S Willett, G Smith; G Paul, S Whybrow, S Johnson, P Connell, N Waterland, A Key, S Davies, C Willett capt.
Reps used: M Huby, B Christian, S Morley, R Davies, C Jeffels, M Hayward, I Mattison.
Doncaster Phoenix: M Jaznakowski; A Burton, C Thomas, B Millar, D Brookfield; Tim Elliott, Tom Elliott; M Roodt, P Atkinson, D Fielding, M Longworth capt, N Germain, D Morris, H Taylor, J Oakley
Reps used: M Liddle, A Kaihau, B Wilkinson, R Roodt, J Stuart
Ref: B Fitzmaurice (RFU)
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