Footholds should each have 5 stages, and when 1 spawns it starts at stage 1.

Foothold - Same as now. Only spawns minor invasions, and is easily soloed

Fortified Foothold - Footholds upgrade to this after around 5-15 mins. Terrain expands slightly with more mobs, foothold becomes bigger and has a basic attack. Minor invasions spawn frequently with a chance of a moderate invasion spawning instead. This should take a small group of 2-3 players, though it can be soloed, but will be very challenging.

Stronghold - Footholds upgrade to this between 30-45 mins. Terrain expands even further and it sort of becomes alive. I.e. when standing on it you gain a debuff, so for example standing on death terrain you have a 'fear' debuff which decreases your stats. Monsters are now upgraded to elites, small destroyable walls surround the foothold and the entrance is guarded by 2 soloable turrets. The idol itself becomes even bigger and is now known as a 'shrine', which has a powerful laser attack. This foothold spawns moderate invasions with a chance of a major invasion (elites). This should take a full group to take down

Fortified Stronghold - Footholds become this between 90-120 mins. Terrain now covers a large area (around the size of a small town, e.g. granite falls), debuff is upgraded with additional effects. There are now 2 layers of walls and around 4-6 turrets defending the central shrine. In addition to elite mobs, their is now a raid-level boss defending the shrine. This spawns only major invasions. Rewards for taking this down is a blue stone. Takes a raid between 15-20 players to destroy.

Citadel - Now we're talking. This is the final stage footholds can upgrade into, and therefore will likely be rarely seen depending on player participation. Footholds upgrade to this between 180-300 mins. The terrain expands to cover an area around the size of meridian or sanctum. Their are 3 layers of walls which can take heavy punishment and will alert the mobs if attacked. The central beacon is guarded by dozens of raid-level mobs and raid-level turrets. There are 3 bosses guarding each entrance. The beacon itself is equipped to deal with a full raid of 20 players alone. Firthermore, it will very frequently send out major invasions with a chance of sending out a raid-level boss to attack a city, similar to the zone invasion bosses (this is very rare though). Once a foothold upgrades to this, a notification is sent out to all the players in the zone and a zone event triggers, with the main objective of taking down this foothold. Rifts will spawn everywhere similar to the invasions we have now. Rewards for taking it down include an epic stone. Will most likely take between 3-4 full raids to take down depending on level.

In order to prevent exploitation of the system and players intentonally waiting for footholds to upgrade to a citadel, there is only a chance that the footholds will upgrade. I.E. once a foothold spawns its not even guaranteed to upgrade after the set time has passed. This chance diminished for each level. Furthermore, rewards for killing low-level footholds will be increased substantially.