"This game could have really
held its own [i.e. be good enough compared to others]
if the playing field
had been a bit more level."
It's allowable to ellipt the word "if" in the cases that subjunctive is used, with "should" or "had": "Please let me know, if you should experience any problem," can be rephrased as "Please let me know, should you experience any problem." It usually sounds more formal than including "if" as usual. Still, a comma should be used at least: "This game could have really held its own
, had the playing field been a bit more level."