If you design maps so that the bulk of resources are away from the base, fighting will have to start early and fast and there will be no time for turtling.

You could make the base building a separate offline step, sort of like building a deck in a card game.

This. Explicitly separate the phases. Every player starts out with a set amount of resources and before the game proper starts, is allowed to put together a base, but not move units or attack - they may only place units, which take an instant amount of time to build. When the players are finished, they proceed to the next phase, which is the actual battle. Basically you'd skip through the early game while removing the need for reflexes and set build orders through that phase.

And actually prevent base building during the battle phase? It's an interesting idea - allowing you to repair (and maybe replace) damaged buildings would be nice but separating the phases could work. You set up your defenses and resource gathering and this will play a part in how effectively you can build troops.

As a side-effect, building your base defenses first will automatically force players to build bigger armies because initially, the defenses will be massively over-powering.

You could make the base building a separate offline step, sort of like building a deck in a card game.

This. Explicitly separate the phases. Every player starts out with a set amount of resources and before the game proper starts, is allowed to put together a base, but not move units or attack - they may only place units, which take an instant amount of time to build. When the players are finished, they proceed to the next phase, which is the actual battle. Basically you'd skip through the early game while removing the need for reflexes and set build orders through that phase.

And actually prevent base building during the battle phase? It's an interesting idea - allowing you to repair (and maybe replace) damaged buildings would be nice but separating the phases could work. You set up your defenses and resource gathering and this will play a part in how effectively you can build troops.

I didn't have that in mind - I was just laying out a way to "skip through" the early game in a balanced way - but that is in an interesting idea. Though, if one messes up their initial base setup it could encourage ragequits and other poor sportsmanship in multiplayer matches.

Also, I wouldn't be terribly surprised if there's already custom scenarios for established games that do something like this, since it's something that could easily be done in something like StarCraft's scenario editor.

I'm kind of reminded of Netstorm, in that every player had their own unique floating island that was of a persistent shape from game to game, that was basically a plot of land for base building. Though in that game, you still had to build everything from scratch at the start of every game, so the analogy breaks down shortly after.

As for strong early game defenses, games like Age of Empires did this somewhat well, in that there was a little alarm bell icon that could be pushed, and all the peons would run into the town hall and shoot arrows at attackers, and all early attackers did very, very little damage to buildings. I think SC2 borrowed it for one of the Terran upgrades, but AoE it was universal and there from the beginning. It wasn't perfect at stopping early rushes, but it did help. Doing something similar where perhaps all buildings have built in defenses that need to be overcome by either a huge amount of level 1 units, a decent amount of level 2 units, or a small amount of siege units, could all contribute to making the game feel more turtle-ish or slower.

You could also take it back to Dune II and go in a different direction. Make building quick and maybe even free, but force the entire base to be on an irregular map object (like the stone "islands"). You don't even need a resource to spend, necessarily, because your real resource is space. You may only have 2 or 3 spaces large ennough to fit those good big buildings.

And maybe when a building is demolished, most of the spaces are usable again, except for a few. So you can rebuild, but not the same thing.

Remove building times and come with a finite amount of resources perhaps?

...Also, you can sequence things as in Ground Control where reinforcements are shipped in waves...

...What if all units are AI controlled and behave like a mob?

This makes me think of tug of war games that I've played on starcraft 2 custom maps. You make buildings instantly, then mobs of AI controlled armies attack the enemy. It takes a while for the mobs to push to the other side, giving it a tug of war feeling as the front line is slowly moved. Sometimes these also have layers of defenses, that do a lot to hold off the enemy in the beginning, but later on armies get too big for the defenses to do much.

Have an early defender’s advantage that diminishes as the game progresses. You could for example make powerful base defenses that are cost effective against early units, but are weak against upgraded or higher tier units like artillery and aircraft.

I think this idea fits well. It weakens early attacks, but still allows a range of aggression, tech, and expansion. The idea is to keep the full range of RTS options, just skewing the balance towards building in the early game, without breaking the balance in the end game. For example, maybe your home base starts with some of these basic defenses, but an expansion would be less defended until those defenses are up. So building an aggressive army at the start of the game might help you to destroy an enemies expansion, even if it can't do much to their home base. Your early game army would also help you to defend your own follow-up expansion.

You could make the base building a separate offline step,

definitely the way to go.

player gets a certain amount of resources for building, or whole buildings to place, before the game starts. then RTS as usual. problem solved - get a beer! <g>.

I like an RTS where you take time to build a well-defended base and a decent army. It's how I play RTS games even though it's not the 'right' way, but one problem is that it makes every game last for hours. I almost split the game in two - 1)build a base 2)battle the enemies

The first part tends to be quite repetitive as well and while some people probably like that, I wondered how I can promote the "big base" approach without a mandatory 1 hour base-building phase at the start of each game?

I do not recommend spliting the game in two. A players base can be made inaccessable for a brief period of time. Giving them a small window to establish their strategy. No other players can see their enemies bases or leave their own. A brief sense of safety will give commanders a chance to plan out their strategy at the cost of a time contraint to do so, limited resources in their personal space, and of course a limited space to fit said structures and units. They will be forced to to make use of limited time, money, and space. An hour is too long. Let the players have the option to choose how long they want to be contained within their base, how much money they have, and how much space.

Maintenance/Limits

Most RTS games with base construction suffer from the same problem, nothing ensures "proper" balance between army size and economic power.

By adding a maintenance system to units you prevent initial rushes to a certain degree, therefore ensuring players develop some infrastructure, and since now all players are on the same page, you can make the base building process faster.

By adding a maintenance/limit system to buildings you prevent super bases from being built.

Of course that all this is pointless if the combat system has low tactical depth.

Taking one hour to build a base and shift to attack mode means that collecting enough resources to build a base takes one hour.

This could mean that resources are too hard to collect, building prices are too high, or resources are diverted to other purposes (like building throwaway units for patrol and exploration). Hopefully the building and upgrading process is not slow by itself; one hour of clicking on the right button at the right time, or the base will be built in one hour and a half, is too horrible to contemplate..

The recommendation to build the base quickly at the start of the game is good because it bypasses all possible reasons for slow building, ensuring that the game begins with well developed bases: no competing ways to spend resources, no resource collection, not even long building times.

I recommend an abstract user interface for preliminary selection of available buildings (with constrained map-specific choices, e.g. 2 barracks vs 1 barrack and 1 shipyard), then placing the selected and mandatory buildings on the map before the clock starts running, then variable combinations of preliminary placement and initial funds for regular building for walls, turrets, roads and other things that need to be installed at specific locations after laying down the main buildings. Strategically, base layout should be very important for defensive effectiveness, or automatic building placement would suffice.