Here is an excerpt from the Imperial library:
In the 2703rd year of the First Era, Tamriel first faced an organized armada of Tsaesci, the so-called "Akaviri Snakemen", and met the challenge with a resounding victory in the Pale Pass of Skyrim. The Emperor Reman was so impressed by the exotic weaponry and battle prowess of his defeated foe that he and his heirs allowed them to rise from prisoners to advisors of the second Empire of Tamriel. Eventually, they became, on the death of Emperor Reman III in the year 2920, the Potentates of Tamriel, and the defeated Akaviri ruled the land for over 400 years.
As you can see they ruled Tamriel for over 400 years. How did they keep the Daedra at bay for so long?