Bad Kitty...

I have a bad kitty, and I don't know what to do about him. We have 2 cats, Diesel and Princess, they just turned 2 earlier this month and they have been living with us since they were about 3 months old.
When they were kittens they were sooo good, they were trained right away and they didn't shed much at all. Then Diesel (the male) started to mark things in my house, it started in the guest room. He marked the bed, the drapes, the carpet, everything! It got so bad that I finally threw out the mattress and started to keep the bedroom door shut. Things started to get better, he went a few months and didn't mark again.
Then last summer he started marking on toys and pillows that were left out on the floor. I finally brought him into the vet and they said it was clinical depression and that he needed to be put on Prozac! I didn't even know they made prozac for cats! Regardless, I started treating his depression with liquid prozac and again things got better. He still had a few incidents but they were small and far between.
In October we moved into the new house. I was worried his marking might start up again, he marked a wicker basket and one of the cat beds but that was it. I kept a close eye on things but he seemed okay. Well last month he marked in our pop-up-crib (I set it up when we had two babies visiting). This was by far the most upset I had ever been. Before it was mainly on things of his, but this was serious to me. I can't imagine living day to day worried about him using a bed or crib as his litter box.
Anyways, this morning we woke up to him pucking in the spare room. Derek went to clean it up and immediately smelt pee. Turns out he has peed and pooped about 10 times in this cat tower we have set up for them. Luckily it was easy enough to carry out and the smell isn't in my carpet. I'm really hurting though. I love animals, but I can't live in a house he has control over. I shouldn't have to keep every door closed so he won't go into them.
If this is depression, what can I do to help it. I have ready every book/article/website and I have tried just about everything. Most people say it is because I don't spend enough time with him! Or because the vet must not have clipped him right (we did have this checked out and they did do it right).
What am I supposed to do. Do I find another home for him, do I live with the fear of him marking my entire house, this sucks. He doesn't have front claws or I could see if living outside would be a better fit. I need some advice. I feel like a failure!