There are possibility to defend Islands,why not make some option how you can defend your towers and barricades? That would make attacking harder and more interesting,you would have to think about best strategies.
I suggest you to make some new improvement system- traps which can be put before towers and barricades.
Slowing traps,Damage per sec traps(fire,poison),normal damage traps,teleporting traps,instant kill traps(only for troopers) and etc…
And of course chance to disarm them using gold. And if you use more gold to disarm them,the less you get at the end ,which makes progress little bit slower for some players