Being a Multi-player game, there is the added aspect of having Friends to play with. Friends provide several different advantages.
Making Friends Edit
There are two different ways to gain friends in Dungeon Overlord. The first is to know them through real life (Facebook). If you invite a friend to play and they accept, then they will join the list of friends at the bottom of the game display. Note that, just because you and your friend are facebook friends and both play DO is not enough for them to appear; someone must be invited to connect up.
The other way is to find a player in game and invite them to become a friend. This will not reveal any information about their facebook profile, other than their profile pic.
Types of friends Edit
There are two different types of friends: friends and trusted friends. For a friend to become a trusted friend, you must click on their name and select "Trust". This will make them one of your trusted friends and will add the text "Trusted Friend" to their name. In order to become one of their trusted friends, they must add you as a Trusted Friend. When you have been added as a Trusted Friend, their background will change from blue to brown.
Benefits of Friendship Edit
You don't have to be official friends with a player in order to do nice things for them. You can always send mail to any in game player. You can also always send resources as well. These actions are taken from the Overworld screen, by clicking on one of their dungeons.
If you know a person through Facebook, some actions give you the option of sharing events to facebook. Currently, those are limited to some high level Room Upgrades and collecting all three daily treats during the Halloween Event. In the room upgrade, they are given the option to click and help you (though it is unclear how the help behaves... perhaps shortening the time to upgrade?) and they will receive a reward in the Vault of their starter dungeon. For the event treats, they can simply receive the reward to no benefit to you. The contents of the reward are decided by whatever triggered it, but are similar in scope to the Daily Chest
Any kind of friendship (regular or trusted) has the additional benefits of being able to reinforce their dungeons. This will send some of your creatures to live in their dungeon to help in defense. You are still required to pay the upkeep however.
You can also use those friends as Taskmaster. This is accomplished under the Details section of a room in one of your dungeons. Having a Taskmaster will give some type of bonus (depending on the room) to your room, and the magnitude of that bonus is related to how active your friend is in playing DO.
It's also easier to send mail to a friend, as you can reach it by clicking on their name at the bottom of the display.
Trusted Friend Edit
If a friend has listed you as a trusted friend, you can click on their name and select "Visit". This will allow you to enter into their dungeons and view their layout. Additionally, you may collect any resource piles for them. Doing so will put the resources into their stores (if they have room). It's a very nice thing you can do if they might not be able to log in soon enough to avoid missing out on resource collection due to filled tiles. If you visit their dungeon, they will receive a notification indicating that you did so (presumably so as not to be shocked that their resource piles are smaller than anticipated).