Striker Jamie Proctor put Crawley on course for their first win in four games with a 25th-minute volley, but midfielder Simon Gillett, on loan from Nottingham Forest, extended City's unbeaten run to six games with a 62nd-minute equaliser.
Cotterill said: "To finish in the top half of the table is a wonderful achievement. It was a great effort from everyone connected with the club.
"The boys never gave up and it's great testament to their spirit. We have already started something and now I have to try to bring other players into the squad."
Cotterill admitted he was eyeing a 10th-placed finish but stressed: "Going from 24th to 12th is remarkable really. All credit to the boys and I thanked them for their effort.
"Now I have a difficult couple of days as I have to re-jig the squad. I've no idea about the budget whatsoever, but there's now a few awkward conversations that I have to make."
Cotterill believes City striker Sam Baldock, leading scorer in the division with 26 goals, is a fully deserving winner of the League One Golden Boot award.
He said: "It's a fantastic achievement and great credit and testament goes to him.
"He looks happy and at home with us and it is our intention to keep him.
"He thoroughly deserves the award as he is top scorer in the division and I am delighted for him."