Hi Tuxu. Welcome to Tremulous!
As someone who has played Empires Mod quite a lot and Tremulous even more, I'd like to help you by comparing the two game styles from my experience.
Firstly, I'd say that there's a whole lot less of the RTS element in Trem than there is in Empires. This is a good thing if you don't want your game ruined by a single bad commander. This is a bad thing if you want more long term strategies and evolving conflicts.
Most public games in Trem have little in the way of tactical building and since anyone can build with the teams build resources (which are fixed from the start and don't diminish) if someone does build the majority of the time this person doesn't know what they're doing and get's complained at (sort of like the commander in Empires only on a more individual basis). There are various measures against this sort of thing, most notably commands like '!designate' and '!denybuild' so that admins can identify who should and shouldn't build. There's also '!revert' for when someone griefs by deconning the reactor/Overmind/spawns etc. (sort of like dumping the CV in the enemy base or off a cliff). For this reason and many more I highly recommend downloading
TJWs Backport to replace your executable as you won't be bale to get admin and certain bug fixes without it.
If strategic gamepley is what you're looking for then your best bet is joining a clan (of which there are many in Trem) or just sticking to a group of friends who you know are able to work with their teammates. Public games tend to be much more of a frenzy and because the enemy gains upgrade points from killing your teammates, the weaker players in your team will undoubtedly 'feed' the enemy which can lose you the game.
Also note that this game is open-source and highly modable and so many of the features I've tlked about can vary wildly from mod to mod (e.g. TremX, MGdev etc).
In my opinion, it isn't the rts element that makes Trem fun. It's more of an added bonus if you have a game with real teamwork and strategy. Most of the time you just enjoy the gripping alien-vs-human combat mechanics which
can be tactical but on much more of a split second basis.