If you don't want a stuffty excuse for a mini empires server, you'd need an admin, or admins to heavily moderate the server and make sure that no one just duplicates death stars all over their island. And chances of finding someone willing to put that much effort into it is so low that its probably not worth looking for.
They can still all end up Afk leaving you vulnerable.
In which case I always leave a bare patch of land with a small military tower nearby, and if 'noobs' attack I just click the top of the tower; grass was a hatch, opens up, huge ICBM (inter-continental ballistic missile) shows off in its silo, click tower again, missile fires with sounds and FX, then just RP from there...
Unless strictly speaking its medieval on medieval. Then I recommend looking into a little something called
Greek fire.
Set up a few trebuchets along your coast (not like they NEED to be in storage), unless you can use 15th century weapons, which means cannons. Then your gonna wanna make
simple mortars, and for a list of possible
projectiles, then you can always check wikipedia.