Minefields!

I'd like to see a mechanism for systems to gradually deploy a minefield as a rudimentary defensive layer, capable of reducing a single fleet by 10-30% of full health.

I envision this as initially being a Tech Tier 1 defensive structure.  Once built, the mine deployment structure would slowly generate the field over 5-10 turns while no enemies are blockading the system.  Mines would need to be rebuilt after having been triggered by an enemy fleet. Partially deployed fields would have reduced effectiveness, but still function.

As to when the mines should be triggered, I think it could be handled in one of two phases.  

1. On blockading or invading a system, since that is when the fleet has engaged the system as a hostile.  

2. Triggered in battle, during the arrival (and now, defensive bombardment) phase.  This same phase could also incorporate the ideas from other posts about ground and space-based defensive arrays and missile barrages.  The idea here is that a well-developed system should be able to support a defending fleet, and this would be a visually exciting addition to the entry phase of battles.

To avoid a loss-leader approach that would entirely negate the defense usefulness of this feature, there could be a detonator switch that the player triggers as needed.

There could be research tied to these that would add some variety (Armor/Shield-piercing, stealth, anti-personnel, EMP-effects, even a friendly-fleet restorative or boosting effect), likely lassoed to the current Strategic Resource battle modules, or they could simply be upgraded by expending a significant amount of Strategic Resources (50?), to keep the damage relevant as the game progresses

Finally, for the sake of balance for a would-be conqueror, a support modification could be added that protects attacking fleets from mines, making room for a new battle role for support class ships - mine sweeper.