What is the overall goals of people who criticize you in front of your co-workers, clients, and boss?

You are right when you say: "I always considered the Senior level person to be an ally...on the same team ...". That's how it should be!!!... but it doesn't sound like this person is cut out to be in a senior level position. To me it sounds like they're not a confident enough person to stand behind the work being done when it is questioned by the client. You guys are the professionals and have done your work the way you did for a particular reason and he should explain WHY you did it instead of knocking it in front of you. That is VERY unprofessional. He/you can listen to the client's comments and take that into consideration and make necessary changes to accommodate but putting you and your work in the hot seat in front of the client is very wrong! This is something he should have said to you before the meeting, or if he didn't notice before....then after the meeting in PRIVATE. Perhaps this is something that you can bring up to him in a private meeting of your own. If it doesn't fix anything, it might be time to look for another job. One where they will appreciate you! Good luck!