Is it optional to dress up?

Yes

You can dress up and it's a lot of fun, the employees get a huge kick out of it. My whole family dressed up and I think we got better service and noticed more by our knight.

Of course! Very very few people do, but highly encouraged. Beware of bringing any weaponry of course...

Nobody dresses up (except the characters) in costumes if that's what you mean. Dress casual.

Many do and go here to celebrate birthdays, anniversaries, and other special occasions. I had a engagement party here.

We dressed in Rens Faire style & had lots of fun.

I have never seen adults dress up but they do sell stuff for the kids.