PVP (Play versus Pay) is an interesting idea, but it's poo.
Suggestions for DoF's PVP to improve.
Fix the exploit where you get an hour of attack protection for only paying 2000-6000 gold and not engaging in combat. This is known as "paying out"
Fix the bug where attackers get protection for giving the defender protection. This leads to cries of "No one is pvping!" because all of the attackers keep around their safety net while searching for Play vs Pay.
Fix how overpowered not having walls is. Currently players are abusing only having to fend off wolves without using trebuchets to blow up their own buildings -- oh and they don't have to pay 2000 gold for protection. So... yeah... add an incentive for people to build walls.
People might enjoy a tad of risk while building their city if they give it a try instead of sitting around with camps until they get a dragon and uninstall.
And even after the the new units that try to encourage sieging I believe people are still intimidated by the 20 extended-range-trebuchets and 30 popcap advantage along with the defender's ability to make units in battle. I'm not even sure if it's clear to the person attacking that they gain a lot more crowns if they win.