Platt put Colchester ahead in the 67th minute. There was a slice of good fortune about the goal which stemmed from a rebound after a defensive clearance. The ball fell to Platt six yards out leaving him the simple task of sliding the ball past Adam Collin.
The home team were level within a minute when Jason Price wriggled his way into space to the right of goal and pulled back for substitute Marshall to clip a first time shot high into the net from eight yards.
Carlisle went in front when Marshall played the ball in towards the near post where Price was on hand to supply the vital touch.
Platt was denied a second for Colchester in the closing stages when his close-range effort cannoned off defender Evan Horwood and somehow lodged in keeper Collin's grasp as he lay on the ground.