currently the top 25 coals have level 20 capitals , whilst the majority  of the other coals are lvl 5 capitals . these are nearly all empty of troops /these undefended capitals are looted of their flags  routinely.

any attempt to upgrade a capital is met with overwhelming force by one of the top 20 coals.

the balance of power between a coal with a lvl 20 capital and one with a level5 is huge.

in other areas plarium has accepted there are power levels , eg the use of leagues [coalition clash  etc

a system where by you could only attack a capital within 5 levels of your capital, would enable other coals to upgrade their capital above level 5 and encourage them to place defence troops in their capitals.

realistically 80% of all coals dont even bother about their capital as its impossible to upgrade them in the present time