Why the 18-inch gun points to a shipboard requirement

Were the British 18-inch guns developed primarily to replace old 100-ton coastal guns at Malta and Gibraltar?

No. Their design pattern shows a shipboard requirement, although a naval gun could later be adapted for land service.

A ship gun and a weapon designed solely for a fixed land position face different engineering constraints. Mechanisms, breech arrangements, available space, and stabilization all help indicate the intended use. The 18-inch gun's design is interpreted as evidence of a shipboard requirement, even though it could also be used ashore. Stabilization alone does not account for all the differences. The proposed replacement of the old coastal guns at Malta and Gibraltar therefore does not explain the design as a purely land-based weapon.