I've always loved visiting old castles, but this is the first time I've been able to scramble underneath one - through tunnels that were used during a siege in the sixteenth century. There are actually two tunnels, although they meet underground: the mine constructed by the besieging army, who wanted to plant bombs beneath the walls, and the counter-mine hurriedly dug by the defenders. It's really interesting to see the difference in construction between the two. The attackers took their time, creating a broad route down with gentle steps cut into the earth. Conversely, the defenders were responding at a rush and without full information - going by the sound of the attackers' tools, they made several false starts (which you can also see in the castle), and their part of the tunnel is uneven, narrow, and generally less finished. But it worked, which is presumably all they really cared about.
These days, although it's still an adventure to visit, you only have to worry about passing other tourists in such a narrow space. The entrance to the tunnel has a very low ceiling, so I had to walk in a crouch through the first section. It's damp, cold, and has an earthy smell of clay - quite a change from the bracing sea air aboveground. The underground tunnels definitely make St Andrews stand out from other ruined castles, and I'd recommend it to anyone who's interested in getting a feel for the more military aspects of ancient life.
The rest of the castle was in somewhat poor repair, but with a stunning aspect looking out to sea - which, now I come to think about it, is probably related to the crumbling state of the masonry. There's also an intact bottle dungeon, which you can peer down into, although there's sadly no ladder to allow you to experience it from the prisoner's-eye-view.