Its up some steps to viewing deck 2, which also has a cafe, this is where we left daddy whilst Charlotte and I headed up to the Crows Nest (110m above sea level). This is the smallest deck, glass on 3 sides but with an open roof, you really do get the best views from up there. The lift to go back down departs from deck 2.
Charlotte really enjoyed walking on the glass floor, and we really loved the views across Portsmouth to Isle of Wight, watching the boats come and go.
I was surprised by how small the viewing decks were, on busy days you could be jostling for space at the windows, but that's only as we were comparing it to the Sky Tower, Calgary, Canada. One negative was having to pay for children from the age of 3, a 3 year old isn't going to appreciate it as much as an older child.