Martello Inner stone

Once I managed to climb into this structure things became a whole lot more complex.
The inner section had deterioated quite badly though this was an amazing first for me. Complex tunnels and venting systems enabled difficult penetration of this structure.
A circlular vaulted ceiling built in a combination of brick and stone with staligtites forming from water leaking through the lime mortar allowing recarbonation.

http://www.martello-towers.co.uk/