In the current POS mechanics we live under, the EHP of the tower provides a significant barrier to casual attack. The larger the tower, the less likely it is to be attacked just because. Add in the combinations of guns, ECM, and EWAR that are possible and you have an environment where the variations in DPS and effect create a challenging situation for any attacker that must be considered. In addition, during the attack itself, the tower defends itself in a mediocre way, deterring some casual attacks, but becomes more effective when actively operated. The threat to the tower can come from a variety of sources and ships making the act of defending a tower an interesting proposition and one generally requiring fleet support.
Under the Citadel mechanics, things shift quite significantly to an unbalanced attack and defense equation. The attacker has only one option: Use an Entosis link against the structure. The defender, however, has guns. Those guns need to target one type of ship, an attacking Entosis ship. An Entosis ship, while actively engaging the structure, can receive no remote support and cannot warp. Any on grid support in the form of a fleet is somewhat irrelevant because they cannot take action to help the Entosis ship in terms of EHP which is now the barrier to beat. From a defending standpoint, the Citadel only has to only exceed the tank of the ship in question and the attack is thwarted. Rinse, repeat.
This process extends to other Citadel defense options as well. ECM alone will make the process so frustrating for an attacker due to the Entosis warm-up cycle that the only counter would have to be a fleet of ships greater than the targeting and jamming cycles of the limited number of slots a tower has. These limited slots are what will make Citadel attack purely a equation to solve much like existing PvE content:
1. How much DPS do I need to tank on any given ship
2. How many ships I need to bring in order to overwhelm targeting and ECM cycles to maintain at least 1 Entosis at all times.
If CCP has set these thresholds too small, a Citadel is a dog with all bark and no bite and can be reinforced by a ship incommensurate to the structure size. If this threshold is too high, the Citadel is literally an impenetrable wall that blaps ships indiscriminately off the field and jams out every attacking ship causing repeated warm-up cycles.
We all know how non-compelling existing PvE content is when the equation to beat it has been solved. This DPS/EHP/ECM threshold is disappointing in a game that has prided itself on complexity and depth especially in what is ostensibly a PvP encounter. It also does not achieve the goal CCP has put forth with the Entosis link itself: grid control. Notice nowhere to this point have I needed to discuss a defending fleet. Citadel combat isn't about grid control until you have exceeded the threshold of damage and annoyance that the Citadel can dish out during the duration of applied Entosis modules. At this point the Citadel has merely become a speed bump and even worse, has set the minimum number of pilots that are needed to engage a Citadel possibly to the exclusion of smaller groups participating in this aspect of the game.
I believe that much of this revolves, quite simply, around the proposed slot layout. It is too simplistic and does not offer a wide enough range of attack and defense options to a Citadel. It simplifies the equation too much.
As a final thought, the asset safety mechanic in Citadel destruction is the final nail in the coffin for wormhole based structure combat. If there is no chance of receiving reward for attacking a neighboring entity, the desire to do so will be limited to grudge matches or lolz. It is already difficult in the POS environment to want to kick over someone else's castle when they frequently will self-destruct assets while you watch, but at least there is a chance. Eliminating that reward will be very detrimental to conflict drivers in wormhole space where there is basically no other motivation to engage a Citadel. Even the practice of attacking structures to provoke a response will be discouraged by the numbers of ships and associated EHP outlined above required to survive a single pilot manning the Citadel defenses.
I do not want to be only a naysayer when it comes to EVE features. I will take some more time to consider options and hopefully have something here to propose. At first glance though, my gut feeling is that Citadels, at least in wormhole space and perhaps elsewhere throughout k-space may need to be Entosis free. While structure grinds for sov were very undesirable for many reasons, the act of grinding a structure to remove that structure does not seem out of line and would also continue to provide a use for higher level ships that currently are losing nearly all their utility in an Entosis based universe.