As suggested by Urban-hacker in discussion #130 , reward players with a badge (or something like that) for finishing a level by playing with self-imposed handicaps. These bades would be rewarded per-level, thus requiring players to retry every level with different strategies.
This would provide a lot of replay value for previously cleared levels, in exchange for relatively little time investment.
Examples of self-imposed handicaps:
Don't use chip X
Don't invert attackers by tapping on them (probably easy to mess up this one with a mistap)
Don't upgrade your chips (probably hard to achieve in every level; maybe something like "don't upgrade above level 3" would be more manageable but still provide a challenge)
Don't fill every available chip slot
Don't sell negative chips
Possess a certain amount of unused bits by the end of the level (this one i don't think is possible, because bits depends on attacker power and value as well as hero upgrades)
As suggested by Urban-hacker in discussion #130 , reward players with a badge (or something like that) for finishing a level by playing with self-imposed handicaps. These bades would be rewarded per-level, thus requiring players to retry every level with different strategies. This would provide a lot of replay value for previously cleared levels, in exchange for relatively little time investment.
Examples of self-imposed handicaps: