I've talked about this before, but I'd like to talk about it again. Constructive terrain games. That is, games where you build a base/s. Whether it's Starcraft or Evil Genius or Civilization.
In thinking about these games, I've tried very hard to come up with the methods used to make them interesting and challenging. The challenges I'm most interested in are those related to building the base, rather than those related to putting the base in the right spot on the map. So I'm not going to talk about map-tile resource allocations or complex topography much. I'm going to talk about the parts where the player twists himself up in a knot of his own making.
There are three kinds of challenges that relate to this. Most of these challenges are expressed via semipermanent, fixed structures that you can place.
The first is the arbitrary challenge/facility. This is a situation where the location on the map doesn't matter. For example, placing a research facility: no matter where you place it, the research happens at the same rate and is just as applicable.
Arbitrary challenges are often simple treadmills to extend the gameplay. Plunk this down to steadily unlock stronger soldiers, if you have the money for it. However, they can also serve as a gating mechanism, unlocking additional facilities over time. This forces the player to build up their core base with a complexifying crowd of fundamental buildings, making the placement of advanced buildings somewhat tricky. Advanced players will often leave empty zones in their base as they build, knowing that they'll want to put advanced structures in safe places well inside their core base zone.
Arbitrary challenges are usually made a bit trickier simply by resource allocation. An arbitrary challenge facility will siphon away valuable resources, and your strategy requires you to balance the upgrades/abilities it offers against your more direct unit/facility purchasing.
They can also be made trickier by having very awkward shapes. This is especially useful if you're playing a contiguous base game, where every facility must be butting up against another facility, door-to-door. While any facility can have an awkward shape, this is the best type to make awkward, as the other types should be reasonably easy to place anywhere.
Speaking of the other types, in addition to arbitrary challenges, there are ZOC challenges.
Zone of control challenge facilities are those that have a radius of effect, or whose effect diminishes as range increases. For example, you put the lumberjack shack next to the forest so the round trip is tiny. You put the turret on a path that the enemies are likely to use.
A subset of ZOC are those that are required in order to place your own facilities, such as Protoss pylons or Dune's concrete. I'll call these "extend ZOC". These are worth mentioning as their own distinct type simply because if they exist, they are critically important parts of your strategy.
ZOC challenges sometimes relate to fixed map resources, in which case the point is to build a ZOC facility very near the resource. Combined with complex topology and extend ZOCs, this can actually be quite a challenge. Protecting a particularly remote facility may also be quite a challenge.
Other ZOC challenges relate to non-fixed map elements, such as traveling enemy soldiers or random resource spawns. Knowing the paths/likely positions of these elements is critical, and there are often "layers" of these kinds of ZOC challenges. For example, this turret protects against ground units, that one against air units. You place them assuming the ground units will go around the mountain and the air units over it: the ZOC has very different weights because they are addressing different kinds of non-fixed map elements.
The third ZOC challenge type is when the ZOC relates to your own base. Extend ZOC facilities are one example, but there are often other buildings such as police stations, repair droid ports, energy broadcast stations, sewage treatment facilities, and so on. Games with a lot of my-base-ZOC challenges are typically not heavy combat, but are instead heavy on building.
After arbitrary and ZOC challenges there is a third type: pathing challenges.
Pathing challenges are when the point is to direct units or resources along specific paths. For example, walls of sandbags to force the zombies to come along a torturous maze. Or electrical lines to carry power to your distant buildings. Or roads.
Pathing challenges sometimes intertwine with arbitrary and ZOC challenges. For example, any contiguous base game makes the whole base into a pathing challenge, where you may have to be quite clever with your base design and where you extend to. Pathing challenges may also interact with ZOC challenges, especially if you are putting down a path for enemy units: the ZOC facilities will cover the path and bombard the enemy.
However, the deepest kind of pathing challenges are those that change over time, or butt heads with changing situations.
For example, in Evil Genius, you have a contiguous base. Your minions walk through the base in a strictly simulated manner. The pathing you provide in the early game will rapidly become unwieldy as you struggle with larger numbers of minions and desperately trying to keep travel times short rather than putting that new facility a mile away where there's actually room.
Oldschool RTS players will probably remember difficulties with base placement in old games: you would build a base, then realize you had cut the travel path, and units couldn't actually get out. Another example.
This is relatively rare with things other than travel paths, though. Electrical lines can always carry infinite energy, water pipes can always carry infinite sewage, and so on. Still, that's just convention, and it can be bucked.
Another fun complexity to add to pathing challenges is to have multiple kinds of paths that have to coexist. For example, roads and railways. Or even just local roads and highways. Underpasses and overpasses become common, and as the situation changes, the roads inevitably become too limited and narrow. This is especially common in train games.
There are plenty of other ways to make pathing challenges interact with each other if you want.
The point is, if the game wants to let the player twist himself into knots, the game has to allow the player to build his own base, and make that the fundamental restriction on his future expansion.
The core elements of that are:
A) Gate facilities, such that you have to build certain buildings before you can build others.
B) Use ZOC (and possibly travel time on travel paths) to force the player to consolidate.
C) Use travel paths with load limits and steadily increase the load. Optionally use multiple kinds of travel paths that can't perfectly coexist.
There's a subcategory of games where you build your base for another kind of objective entirely. For example, the self-styled bases of Minecraft players. Technically, they use these same kinds of challenges, but the weights are very odd and often selectively imposed. Not every base has to have water, for example, so the water pathing challenge is often simply ignored.
These "erratic challenge" systems are another topic for another day!