Substitute John O'Flynn grabbed the stoppage-time winner after Jacob Blyth gave the Cobblers the first-half lead only for Scot Bennet to equalise before the break.
"You don't very often see games like that where both teams are trying to win it in the final five minutes," said Tisdale, whose team are now fourth.
"We just kept going with intensity and played with a sharpness to our breaks and to our counter-attacks.
"Northampton put enough into their game to feel they deserved to win it but we deserved the opportunity to win it because we kept going right to the end.
"We both deserved to have that chance and it went our way. I thought Northampton played really well, in a lot of respects they were phenomenal.
"They were probably the best team we have played all season but we kept going to the end and took our goals well."