Screeps PvP is biased very heavily toward the defender. Being able to achieve an unbeatable defense is one of the stated goals of the game mechanics.
I personally don't see a problem with this, I think it adds a stability factor to the game. However, a virtually unbreakable defense should be the product of clever coding solutions and not the sheer amount of time you've been playing the game. For example, o4kapuk held his room against an incredibly intense attack with an average wall hits of 6m.
One player suggested a max hits of 50m, and this seems like a good balance. If players want to invest in additional security they can always add more layers at the cost of space and upkeep.
There is also too much difficulty in permanently removing a player from a room. The downgrade timer is just way too high. To downgrade fully from rcl8 takes 405000 ticks, which amounts to 21 days with the current tick times (14 days with more ideal tick times).
I realize the devs want to accomodate players who don't check screeps every day, but this needs to be balanced with the goal of making PvP realistic. I think a simple solution would be to make attackController much more potent than it is, perhaps by a factor of 10.
These are two relatively minor changes that would go a long way toward making PvP in the game more balanced, yet still provide a reasonably favorable situation for the defender. I think there might still be some additional balancing that is necessary. Too many players just do not engage in this aspect of the game because it is unreasonably difficult between more powerful players.