Second Idea:
Coastal Guns-
A4/D4/M0/C15
Coastal guns may fire at warships in adjacent sea zones at a range of one. They may be used defensively or offensively. However surface warships may return fire and destroy coastal guns.
The may be built on any territory with a coastline or island.
The limit of guns per TT is the TT value, a limit of one is permitted on coastal TTs and islands that have no IPC value.
They are considered a immobile unit that can be destroyed.
When encountering an amphibious assault, it negates enemy shore bombardment as the attacking force must engage it as if it were a naval battle.
They are not captured as they are considered an immobile land unit.
They may be taken as a casualty, but do not participate in standard land battles.
They cannot be strategically bombed.
They cannot blockade a SZ i.e. they cannot stop the building of ships even though an enemy TT may also border a SZ within range of the guns.