Here is my idea on how Purges should go. This system should solve all but the problems of purges spawning in the wrong place.
First, get rid of the timer and randomness. Purges should spawn on a per- clan basis, not a random timer for the whole server. Change the meter to have three tiers.
After the first marker you reach, blowing a special horn will immediately trigger an attack on the set of foundations nearest to the blown horn. This purge will be creatures or humans incapable of throwing fire orbs, difficulty one to three.
The second marker, the same conditions apply only this one will spawn a difficulty four to six purge, with fire orbs possible and with a higher chance of rare thralls in human purges.
If you allow the meter to reach the maximum without summoning a purge, then the next time a member of the clan logs on, the game will spawn a maximum difficulty purge within 30 minutes, wether you remain online or not, at a random set of foundations belonging to that clan.
In all cases, only clans being purged should have structure/thrall damage turned on in PVE modes.
This method maximizes player control over their experience, incentivizes players to take on the higher challenges, and keeps the risk if you try to avoid purges entirely.