The Battle for Wesnoth is a free, turn-based tactical strategy game with a high fantasy theme, featuring both single-player, and online/hotseat multiplayer combat. 

"Twelve years ago, David White sat down over a weekend and created the small pet project that we know today as The Battle For Wesnoth. At the time, Dave was the game’s sole programmer, working alongside another person, Francisco Muñoz, who produced the first graphics. As more and more people began to contribute in the years that followed, the game grew from a tiny personal project into the extensive one we know today. If you read the credits, you will find hundreds and hundreds of names who have dedicated tens of thousands of hours contributing toward Wesnoth’s continued development" http://www.wesnoth.org

There are two versions currently available:

• the stable 1.12.x series. For balanced and stable gaming, it is recommended you use the latest version of the 1.12.x branch. http://www.wesnoth.org/start/1.12/
• the 1.13.x development release. Version 1.13.x is the latest development version, boasting new exciting features. However, there may be occasional bugs or performance problems in the development versions since heavy changes are taking place all the time.  This version is recommended for coders and campaign developers, as well as those who want to preview the future of Wesnoth.

In terms of security and stability the best option is always to install from the official repositories. Unfortunately as at Nov 2015 the version at http://community.linuxmint.com/software/view/wesnoth is still stuck with the old 1.10.7 version from 2013!

The next safest option is to download the source code and compile it yourself. There are instructions for this complex procedure at http://wiki.wesnoth.org/CompilingWesnoth. The last resort is install from a PPA.

To do this use Ctrl+Alt+T to open a terminal and then cut and paste the following:

To install 1.12.x Stable version

sudo add-apt-repository ppa:vincent-c/wesnoth

sudo apt-get update

sudo apt-get install wesnoth-1.12

To install 1.13.x Development version

sudo add-apt-repository ppa:pkg-games/wesnoth-devel

sudo apt-get update

sudo apt-get install wesnoth-1.13

CAUTION

Hammer459 is right to warn about the dangers of unverified PPAs. I suggest looking up any PPA before use on https://launchpad.net/ubuntu/+ppas to verify that it is hosted by someone reputable.  In this case you will find that the 1.13 PPA is hosted by the Debian/Ubuntu Games Team and the 1.12 PPA is hosted by Vincent Cheng, a member of that team and also a Debian Developer. So these PPAs are done by the same people who do the official packages and follow the same quality guidelines.  But not all PPAs are this trustworthy so please get into the habit of checking first.