Slow, boring, unimaginative.

If you want to play something newer, check out Homeworld: Deserts of Kharak while it's still on a 67% sale. Also, ALL the Homeworld games aside from that are space based RTS titles. The great thing about Homeworld? You have complete freedom in all directions to strategize and attack. It's to your benefit! It's also a legendary series from the age of Homeworld.

https://www.gog.com/game/homeworld_deserts_of_kharak
https://www.gog.com/game/homeworld_remastered_collection
https://www.gog.com/game/homeworld_emergence

If you like Homeworld and it's space RTS setting you can also check out the following:

https://www.gog.com/game/haegemonia_gold_edition
https://www.gog.com/game/nexus_the_jupiter_incident
https://www.gog.com/game/orb_offworld_resource_base

One of my all-time favorites from the past is Battle Realms. Has it's unique twists to the genre even to this day, has a very fun campaign (you can be good or bad), and it is an RTS title:

https://www.gog.com/game/battle_realms_winter_of_the_wolf

There is also Ashes of the Singularity. While the original game was here, and is on my game shelf, it seems to have merged with Ashes of the Singularity: Escalation. If you enjoy it there is also some DLC to pick up.

https://www.gog.com/game/ashes_of_the_singularity_escalation

Empire Earth II will feel more complete than Empire Earth 1, but they both have their perks. Empire Earth III was a mistake to many people, and for good reasons. Check it out if you haven't played them.

In fact, Empire Earth would be the closest "Age of Empires" style games here that are classic titles as well.

https://www.gog.com/game/empire_earth_gold_edition
https://www.gog.com/game/empire_earth_2_gold

There is also Empires: Dawn of the Modern World. This is also a game that may remind you of Age of Empires in some ways, and it is an RTS:

https://www.gog.com/game/empires_dawn_of_the_modern_world

Total Annihilation is a favorite of many, and it also has another game (not a sequel), Total Annihilation: Kingdoms. Both are RTS games.

https://www.gog.com/game/total_anihilation_commander_pack
https://www.gog.com/game/total_annihilation_kingdoms

The 8-Bit game series are all RTS, and there are currently 3 games here in the series; Hordes, Armies, and Invaders

https://www.gog.com/game/8bit_hordes
https://www.gog.com/game/8bit_armies
https://www.gog.com/game/8bit_invaders

There is also the Anno series which is categorized as real-time, only simulation instead. There are currently 4 games here ranging from 1998 to 2010 in terms of release:

https://www.gog.com/game/anno_1602_ad

You can also check out the KKND series of two games. Both RTS titles:

https://www.gog.com/game/krush_kill_n_destroy_xtreme
https://www.gog.com/game/krush_kill_n_destroy_2_krossfire

There's also the Army Men RTS: https://www.gog.com/game/army_men_rts

Perimeter is also an RTS:

https://www.gog.com/game/perimeter
https://www.gog.com/game/perimeter_emperors_testament

The Settlers series has a lot of games here. They're fun and are considered RTS. Here's one of the many in the series:

https://www.gog.com/game/the_settlers_2_10th_anniversary

Hope this list helps!

Also, if you click on the "real-time" tag on any game it will show you a search for real-time. Here is the link to that search. You may find games you like that I didn't mention:

https://www.gog.com/games?sort=bestselling&search=Real-time&page=1

Also find games that aren't on here in the community wishlist if you'd like to see them up to date and DRM-free, such as Age of Empires (classics or remasters), as well as others that are good.

I recommend Armies of Exigo as a good RTS that's not here, but there is also Rise of Nations, and Rise of Nations: Rise of Legends

As an avid Age of Empires player, I second the Rise of Nations suggestion, if you haven't played it yet. A pity it is not available on GOG.

How about something different for a change of pace? In AI War, you start as the underdog against an all powerful opponent. You have to balance between gaining power and avoiding drawing too much attention to yourself. In my opinion, it has one of the very few RTS A.I. opponent that's interesting to fight.

https://www.gog.com/game/ai_war_collection

The game is on a much bigger scale than most RTS games, with hundreds of units on the field and has many many years of content available now. The downsides I see would be the average graphics (if you care about that) and the long duration of campaigns.

If you ever played Total Annihilation, learning AI War should be pretty quick and painless. If not, there's a very detailed tutorial campaign that can teach you the basics. As a bonus, the game is quite configurable, allowing you to adjust the pace and difficulty to your liking very easily.

STRONGHOLD CRUSADER HD is pretty good.It's the best RTS games I've ever played.

Not really. A mission in Warzone 2100 can march on for an hour or two, especially if you have to do the research yourself.

Hmm, no love for Supreme Commander? Best RTS, considered by many. Unfortunately it's not on GoG.

Is the GOG version (offline installers) now updated to the latest version? Last time I installed the game it downloaded extra files through the auto updater. The explanation of GOG was that the developer threw out many updates per week and it was too costly to build new installers in these short sequences they appeared. Now the game is a bit older. Are there still so many updates?

No, the base game still have some updates available on first launch, at least for the Linux version.

Best traditional RTSes :

- Dawn of War series (the overly brutal and self-parodic wh40k universe makes a good material for that)

- Men of War series (fractally detailed, in the sense that you can zoom down to any unit's inventory and turn a large scale battle into a rpg depending on your personal focus)

- Homeworld series (visually splendid, cleverly 3D, and featuring some of the best videogame music ever)

If it qualifies at a RTS :

- Creeper World series (trying to defeat a sentient liquid mass instead of individual enemy units, somehow feels even more oppressive)

How about city builders? Simcity, Caesar series and so on, they seem to be mainly RT and the difference between RTT and RTS is rather fluid (tactics // strategics).

The X3 games can be played as RTS with you sitting in your flag ship (maybe a tiny one) and ordering your fleet around. (Hey, just trying to think outside our solar system here.)

Come on, I didn't even mention Steamworld Heist THIS time.

Day ain't over yet.

From a quick look at the forums, it seems the GOG installer is on version 7.061, instead of the latest 8.024. They were right that it was pointless to try and keep up with the release rate of Arcen (those guys are ridiculously fast), but now that AI War 2 is on the way, things are more quiet for the original game. Might be time for GOG to catch up.