As was highlighted to me recently, one does not need an actual Plantation to raise plant workers.
Why would not the same be true to produce soldiers. Get a construction permission, haul in some workers and start training them to be soldiers.
If I were to be honest, I understand the need to develop land battles and a place for them but, what the game really needs is a way for your ships to attack ports, if for no other reason, to soften them up in a pre-invasive action.
Realistically, a port must stand by and watch the enemy unload soldiers into your plants port and can perhaps toss an empty beer stein at them.
To be clear, it is in my current way of thought, an idea too soon for its time but necessary after the invasion can take place from an initial port attack.
Something to consider in Plantation warfare, a nation encourages its acreage to be rented to outside nations ( at least in ours) as it is a huge source of outside revenue.
What can a port owning nation do to protect its plants and provide security for its renters ?
In a perfect world, threaten the offending thief's nation with port bombardment or just s**k it up.
If your port was to get to be known for vicious attacks on its acreage, what effect will that have to future renters ?