Despite its importance as a border fortress, there is not a lot left of Berwick castle, because it was inconveniently occupying the site deemed most suitable for a railway station by the Victorians. I have always admired the confidence in progress that led them to feel no qualms about knocking most of it down. A fine view of the only surviving outer wall can be had from the northbound platform of the station. It continues down the hill to the river, as seen in this picture.
If you want to know about the history of the castle, read Jim Herbert's blog, Berwick Timelines (linked to in my sidebar). It's more his sort of thing than mine. He has all the dates and facts about the history of Berwick at his fingertips.
These rooms on the lowest level of the castle, used by the soldiers on duty to keep warm as best they could while keeping an eye out for the enemy, are the only rooms to have survived. I always find it a little creepy when walking past them to the river bank, as shown here. The atmosphere in the tunnel is dark and dank even in the summer. The local drunks treat the cells as convenient waste disposal chutes for their cans and bottles, and the metal grilles installed by the conservation bods make it impossible to reach and remove them. Yes indeed, life goes on.