"After a few games, I realized that whenever the ball crossed the mid-line to our side of the field, the defense, stoppers, sweepers, and fullbacks, would always move up towards the ball, leaving the goal almost completely open."
After "defense" follows a list of three positions that describe defense. This list is a nonessential phrase, meaning that removing the three positions does not change the meaning of the sentence. Therefore, "stoppers, sweepers, and fullbacks" needs to be separated from the rest of the sentence by commas, both before and after. The comma after sweepers is optional as long as you think "sweepers and fullbacks" will not be interpreted as one thing. I also added a comma after "ball", because "leaving the goal almost completely open" is also a nonessential phrase.
A semicolon can be used to separate two complete, related thoughts. For instance... "I was late for school today; my teacher gave me detention." Notice that a semicolon can be replaced with the word 'and', or the two halves can be made into two sentences.
A colon can be used to indicate the beginning of a list. For example... "I bought groceries today: milk, coffee, eggs, and flour." Usually this is used only when the end of the list is also the end of the sentence.
Your sentence could be reworded to include a semicolon. This also removes the possibility that "defense, stoppers, sweepers, and fullbacks" is interpreted as a list of four positions. For example...
"After a few games, I realized that whenever the ball crossed the mid-line to our side of the field, the goal was left almost completely open by the defense: stoppers, sweepers, and fullbacks."
I hope you find this helpful.
~Louis
Note: My father has been an English teacher for 39 years. I was raised to speak and write with flawless grammar. I can personally assure you that all information I have presented to you is accurate. I highly recommend that you take advantage of my help, because many of the suggestions all ready posted here are inaccurate.