The prosecuting authorities (CPS, RAFPA etc) work to a common standard in deciding whether there is sufficient evidence to bring a prosecution. There was clearly sufficient evidence to bring a prosecution in this case. Whether the defendant is guilty or not is a matter for the court to decide.
As has already been mentioned, even if the defendant is innocent of indecent assault, he arguably showed a lack of good judgment, in entering the alleged victim's room via the window and climbing into her bed, all, by his own admission, without invitation. He cannot therefore be totally surprised to find himself on the wrong end of a sexual assault allegation.