Rome: Total War (as it was then called) was the third game in the Total War series and was released in late 2004. It immediately became the biggest-selling game in the series and its graphics engine even powered a historical re-enactment TV show, Time Commanders. Rome's success was all the more impressive given it launched at the same time as Half-Life 2, World of WarCraft and Grand Theft Auto: San Andreas and still managed to be a huge success.
Rome: Total War introduced series staples such as 3D battle and strategy maps, enhanced UI and finer control of units. It also had features oddly missing from later games, such as the ability to view your cities at will in 3D to see how they looked in peacetime with civilians wandering around, and that sites of interest on the campaign map would appear on the battle-map if combat erupted in that area (so if you fought near the Pyramids, the Pyramids would appear on the actual battle map, even though you could easily play a hundred games with that eventuality never arising).
Rome: Total War massively expanded the scope of the series beyond any game before or after (at least until the 2016 release of Total War: Warhammer) and is still-regarded by a hardcore contingent of old-skool fans as the best game in the series.
The remaster upgrades all of the graphics to 4K standards, reworks the UI to something a bit more contemporary and will also take full advantage of modern hardware, overcoming hardcoded engine limitations that mean that both Rome and its successor, Medieval II: Total War are unable to fully use modern amounts of RAM and VRAM (meaning that Medieval II in particular can still chug even on modern machines with preposterously more power than it was ever designed for). You will also be able do unnecessary-but-cool things like fully rotating the battle map and render the game on ultrawide monitors.
Gameplay will mostly be kept the same - which makes sense as change that too much and you might as well just make Rome III - so there won't be any 3D naval battles or armies magically turning into boats, but there will be some tweaks, such as a new roster of civilisations (possibly replacing the generic "Rebels" of the original) and some tweaks to the old ones. It is unclear if the controversial "Fantasy Egypt" faction will be included or replaced by a proper, age-appropriate Greek-Egyptian faction. Merchant agents from Medieval II will also be available for use in Rome.
The game will include both the Barbarian Invasion and Alexander expansions, the latter presumably retaining the appropriately epic narration of Brian Blessed.
One question is mod support. The games released after Medieval II have severely curtailed modding, allowing unit tweaks and reskins but not the "total conversion" mods of Rome and Medieval II that transformed them into completely new, fantastic games like Third Age: Total War (a Middle-earth mod), Call of Warhammer: Total War, Hyrule: Total War and Westeros: Total War. The official PR copy confirms there will be mod support, but not if total conversions will remain supported.
The remaster is being handled by Feral Interactive, who have handled the mobile remasters and ports of other Total War games for the past few years.
Total War: Rome Remastered will be released on 29 April. Those who already own Rome: Total War will get a 50% discount until the start of June.