Turtling is killing the game.
The game in it's current state heavily favors players who turtle behind super high ramparts. Once your code reaches a certain level of energy income there's 0 reason to even build a wall instead of a rampart. This is clearly an issue.
Ramparts should be a weak point in a static perimeter, they should be looked at as gates and portholes that can exploited by an attacker.
As far as numbers go I think both walls and ramparts max hits needs a look, but RAMPART_HITS_MAX should be a significant amount less than WALL_HITS_MAX.
Also I would not be opposed to adding a slow decay to walls to help counter the loss of an energy sink.
As far as implementing it, it's a matter of tweaking those constants and allowing the currently built things to decay down to them. Code should be using those constants anyway so it shouldn't break anything that doesn't need to be broken (USE THE CONSTANTS!).