We ended up at Fort Nelson today. We never planned to go there but we're glad we did.
This museum, on top of the scar in the hillside above Port Solent, is free to visit and has absolutely loads to see. They fired cannons and we even got a little history lesson about shells and gas masks.
The picture I've posted is actually the detail on a cannon! An amazing piece of artwork on something which is fundamentally built to kill people.
I enjoyed the underground tunnels too.