The Pfhor have taken their battle into the depths of time, and you must follow.
Admist the legends of both the Pfhor and S'pht are tales of time travels, and beings that have granted secrets to other races. And one of those secrets have apparently landed in the hands of Leonardo Da Vinci.
You and the S'pht travel must back in time to prevent those secrets from falling into the hands of the Pfhor, before they can use them to cause a time paradox that will rip the universe apart.
As the only things that have really changed in Tempus Irae is the scenery and some expected map layouts, all of the tips that work through Marathon Infinity continue to work here.
Note that there are no sheild recharging stations in the earth-based levels. There are sheild canisters that do teleport in, but you can only use these once. Whenever possible, try to bypass the canisters until you are absolutely low on health.
A common feature in Tempus Irae is the key/lock combination. These are just like the S'pht access cards that were used rarely in the offical triology.
Switches that have the "L" symbol on them are generally very important towards finishing a level or to find a secret.
There are no spoilers for the network levels. This list is only provided for completion and convinence.