Quite oddly, I was in the process of designing a Turn-Based strategy a while back that had to do with tactics.
It turns out that the time element is important. Having the possibility to carefully plan your move is a big part of this.
Most fast-paced RTS actually measure your ability to take as many actions as possible to optimize the efficiency of your troops within a given period of time.
This micro management gives you an advantage as you min/max damage in a given conflict that is often very centralized. You move units back one rank so they stop taking the heat when their hp is growing low and bring fresh troops forwards to keep dealing as much damage as possible.
Now, if you want to have a more tactical game, you'll want to slow that pace, so that players feel at ease making decisions based on thoughts, and not on instincts.
At the speed of Starcraft 2, you learn to do things at the reflex speed. Your body knows when and how to kite units and you'll do it mechanically without much need from your mental acuity. Your brain responds to a stimulus he understands and simply issues a kiting mode and direction. The simple order to "kite" is actually performed through a lot of micro-management sub-commands that your hand carries out, but its, in essence, very simple to think, but hard to execute.
What you want is thus to focus on complex mind commands that are easier to carry out with your hand.
One way to achieve that is to break down the amount of places you want the player to keep their focus on.
In a general Starcraft game, players will focus on 3 or 4 places at once:
Their home base, their expansion if any, any army they have on the move or opponent army they believe is afoot, and the enemy base they seek to take down asap (either base or expansion). Forces are thus concentrated at very focal points to follow these rudimentary functions:
- Prevent enemies from passing through
- Besiege/Attack an enemy base
- Secure a new base
That's the problem. These three functions, while incredibly complex to accomplish, are at the core of the genre's simplicity from a tactics' perspective.
What you need is a mechanic that will force your units to spread out at optional goals which yield optional advantages.
While it is true that, in Starcraft, controlling many bases at once will yield an economic advantage, it is not crucial. You may spend the game trailing one base behind and still win. Besides, extra bases behave exactly like mining colonies most of the time and don't necessarily need you to defend them if your units are keeping the enemy's invading forces in check.
So, in practice, here is one way to achieve this, borrowed from a recent game I've been working on:
Players have bases, that's a given, and they will seek to expand. I've been using preset locations that shift controller. You could say they are very similar to Dawn of War (Warhammer 40k)'s capture points except that each point has its own specific advantages. Some generate specific resources, others grant advantages, others still give you ability to move your units faster, and yet others allow you to build units you otherwise couldn't. That's still a bit lackluster, but at least it forces players to spread out.
The more you spread, the better your economy, but the harder it becomes to defend each location.
By having different locations, one must also choose which resources he will capitalize on, and what he really needs to have a specific army specialization in place. If you had all of the medieval units at your disposal but chose to play a game of speed, you're likely to want to capitalize your efforts towards getting more horses and material to produce light armor. That would give you light mounted units which, while not necessarily powerful, have the advantage of speed, and thus, can quickly reinforce one another when attacked.
This goes to my second point. Decrease speed. And by speed, I don't mean how quickly units fight and whatnot, I mean actual movement speed. And increase sight ranges for "scout" units.
The idea is that you'll give players the ability to spot enemy movements from afar and think of a way to counter this. Rather than running into one another, it gives the player time to try and anticipate which location the enemy seeks to attack. Its a game of deception too, as the enemy won't necessarily trace a line from A to B and give away their strategy. A patient man will be able to take advantage of this and choose the time and place of the battle so that it advantages them. Terrain modifiers will matter too. You want an enemy force stuck to fight from a river to move slowly across is so that, if your fore-archers catch them unaware, you'll have the uppder hand. Chokes are also prevalent. You want to have natural places to defend. This is all in level design though.
When the engagement finally takes place, you'll want the fight to last longer. Here comes my rant on why Warcraft 1 was better than Wacraft 2 in my opinion:
Buildings were insanely long to kill, and it was frustrating, but at least, it made it so that the concept of a settlement had a more permanent nature.
When buildings are easily destroyed, they become an accessory to your strategy. You build them when needed, knowing they can break anytime, and the investment lost is not catastrophic. You can spread them at will.
When they are tougher (and cost more) you get to concentrate them at places of your choosing that you believe you will be able to defend. In return, they will hold against most attacks and allow you to build defensive positions and acquire the defender's advantage.
The defender's advantage, in turn, forces opponents to consider their tactics more carefully. They can't just repeatedly storm at your gates with numbers because their economy is better, they need to plan ahead how they intend on reducing your advantage. Will they lure out your troops by feigning an attack on a weaker settlement only to come head on and take the settlement from you?
Also, make sure that it takes more hits to kill a single unit, or have "groups of units" behave as one controllable unit. The idea is that it will take a while for units to be killed by the enemy, time well spent towards bringing reinforcements or doing the logistics game.
Logistics are also important. A good RTS needs logistics in some way shape or form. You don't necessarily need to resupply archers with arrows, or troops with food, but you need one key mechanic that will make it fun to toy around with. For example, imagine you can spawn units at every location you control, but can only do so using the resources from that location (as opposed to a general stockpile). That would mean production would come with a risk management component. Do you want to spawn your units closer to the front, thus having a military advantage, at the peril of losing a base and letting the enemy steal a large chunk of your precious resources, or would you wather move guarded wagons along the road and build units at whatever location your largest army currently resides?
You also need a military-specific logistic component, but I'll let you think on it.
Hopefully that helps.