Note 1: Format/empahsis changes are mine.
Note 2: Text quoted from http://www.totalwar.com/community/Romereadme.htm
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Rome: Total War Patch 1.5

Installation

You need to upgrade your version of RTW to version 1.3 before you install the 1.5 patch. If you do not, the patching process will not work and your game is unlikely to function.

This read-me details the main fixes made to the game. Other minor fixes and balances have been carried out.

Campaign Map

- We have fixed the “memory leak” slowdown issue that was seen when playing the campaign map for prolonged periods.
-- We have fixed the Naval battles that were not working correctly in version 1.3. AI-controlled factions will now make proper use of ships.
-- Saved games from version 1.2 will work in version 1.5. A couple of other problems with saved games have also been fixed to prevent crashes.
- The Gauls can no longer recruit Naked Fanatics as they no longer have farming-related shrines and temples.
- Academy-class buildings now give a law bonus, as they do in the BI expansion pack. These buildings now have uses in cities without governors or generals in residence.
- Only Roman factions can hold Games at an Arena-class building.
- Diplomacy has been “tweaked” so that Roman diplomatic activity is more coherent.
- Fleeing armies can now only flee once after a large battle defeat.

Units

- We have added secondary low-quality sword to the phalanx-capable German Spearman Warband.
- Removed inappropriate officer from Rebel Archer Warband units.
- Reduced hit points for German Berserkers from 3 to 2 (matching those in BI).

Battle Map

- Improvements have been made to prevent men spilling around the base of siege towers.
- There was a pathfinding issue that made river battles very slow. This has been fixed.
- Improvements have been made to the AI when orchestrating Skirmish Maneouvres.
- The AI will now use 'fire at will' with idle units so that they throw pila at nearby enemies.
- The AI will now make a decision about throwing pila before charging into combat based on the location of enemy units.

Rome: Total War – Barbarian Invasion Patch 1.6

There are a numerous areas of code that were looked at for the patch fix. The list below is the main fixes that have been made; other minor fixes and balancing changes were also carried out.

Campaign Map

- Generals’ night fighting ability is granted at 4 stars rather than 5
- We have fixed the “memory leak” slowdown that was seen on the Campaign map during extended play
- Loyalty is now available with all shadow factions.
- The faction leader will now not revolt.
-- When a horde is created, horde units are no longer added to existing armies that occupy forts.
-- We have fixed a bug that causes hordes to avoid fighting armies on bridges on the campaign map.
- We have given Gratianus Flavius a unit of Imperial German Bodyguards to fix a missing general's unit bug in the starting positions.
- We have made fixes to settlement tax updating, so that it correctly updates tax policy when changing governors
- We fixed a bug that caused diplomats to gather at extreme southwest corner of map in some cases.
- Fixed crash where settlement recruitment queue for rebellions was exceeding max size.
- The Marriage Event trigger now works.
- There are a large number of changes to the balance of the game in response to the extended play that has been done by forum users.
- Fixed an issue where disembarkation paths were being left on the terrain.
- The Nomad voices have been changed from Greek to Barbarian.
- We have fixed crash where settlement recruitment queue for rebellions was exceeding max size.

Battle Map

- Improvements have been made to prevent men spilling around the base of siege towers.
- There was a pathfinding issue that made river battles very slow. This has been fixed.
- Improvements have been made to the AI when orchestrating Skirmish Maneouvres.
- The AI will now use 'fire at will' with idle units so that they throw pila at nearby enemies.
- The AI will now make a decision about throwing pila before charging into combat based on the location of enemy units.
-- The unit experience upgrade “sparkling” effect can now be turned off using the UNIT_EXPERIENCE_UPGRADE_EFFECT tag in preferences.txt.

Several items in these new patches will likely affect past technical advice. There are likely to be some folks having install and savegame issues, too.

Two things stand out, so far:

Quoted from Patch 1.5 Readme:

This has affected several people at HG. The major effect will be on people with low RAM (e.g., 256 MB or less) and/or small Pagefiles (e.g., 512 or less). These people should see an improvement in playability.

Quoted from Patch 1.5 Readme:

This is welcome news.

However, I will pre-emptively recommend that you BACK UP your saves and personally modded files that you want to keep before either upgrading to 1.3 or upgrading to patch 1.5/1.6. Recommend WinRAR, and archive the saves with 3% recovery record; 400MB can RAR to 40MB.

It is unclear what will happen to existing 1.3 saves and 1.3 mods in the patching. It is my gut feel that CA expects a plain vanilla 1.3.

As for the 1.2 and prior installs, the patching might get messy. CA says that you must Patch to 1.3 first, then patch to 1.5. If you have any 1.2-dependent mods, like MM, then we'll have to wait and see what actually happens. But it really looks to me like anyone with 1.2 and before might need to just uninstall RTW, then begin with a clean slate (that is, a plain 1.2/1.3 install)... this is the Patch method that is being tested at CA this weekend. If you run Patch 1.2 or before, you can get a jump on the 1.5 patch by having a clean & working 1.3-patched install (how to do multiple-RTW installs).

The BI patch path looks the most promising and trouble-free... assuming you have a plain-vanilla BI install already working.

If some people can post their successful/unsuccessful experiences with Patch 1.5/1.6 here, it might help others.

Pro bono publico, dum tempus habemus, operemur bonum
( For the good of the public, while we have the time, let us do good )

The new RTW+BI patches are out, a few minutes ago (thanks to The Shogun for posting it in RTW discussion!).

Hmm, I didn't download 1.3, as I thought it was only for BI. So we'll now need to install 1.5 in order to play online?

Yes. I installed RTW several ways today, and (finally!) got some really good crashes in installs. It seems people better not try and cut corners with the CA recommendation to upgrade to 1.3 first... then 1.5... or else it becomes a mess that you must manual-uninstall yourself out of!