Improvements

Improvements are a card type that apply a unique persistent effect to the lane they are attached to. Currently, all improvements are cross-lane cards, able to be played into any of the three lanes.
 * Improvements are not units, creeps, or spells.
 * Improvements do not occupy combat positions.
 * Improvements last until they are destroyed by effects that specifically destroy improvements.
 * Each lane can have a maximum of 16 improvements. Placing a 17th improvement will destroy a random existing improvement.

Types
Each type of improvement is distinct.
 * Active
 * Must be manually activated. Is ready to be activated as soon as it is played, then goes on cooldown. Has 6 spikes surrounding its icon.
 * Passive
 * Triggers when a specific event occurs or a specific phase begins. Has 3 spikes surrounding its icon.
 * Continuous
 * Provides an ongoing, always active effect to the lane. Has no spikes surrounding its icon.