Subtle design decisions can lead to profound changes in player behavior and movement. Some of these can be good for your map, while others can be bad. For this article, I want to focus on both symmetrical and asymmetrical Capture the Flag.
As we mentioned earlier, making a base with too many ingress paths can make the base feel indefensible and lead players to camp the base, even for those that would prefer to push out their defense line. For aggressive players, encouraging camping style defense can be a negative experience, because they want to engage faster and more on their terms.
At the heart of this issue is the ability to play offense while needing to play defense. If the map is designed where the enemy flag is exposed across the map, then a player on red team can stay back at his base to defend his flag, while having some offensive play helping his team take the flag from across the map. For example, a sniper from red base supporting his team mates as they take the blue flag by sniping the blue team as they try to protect their flag.
Narrows is an excellent example of this. Snipers on either end had direct line of sight of the other team's flag, while at the same time being just footsteps from their own. Red sniper at red flag could clear the blue flag of defenders for his team, while at the same time remaining at red flag to defend it. Red sniper is able to enjoy engaging as offense while fulfilling the role of first effort defender for his flag. This is critical to making the game fun for as many of the players as possible. This is why you tend to see flag stands on top of bases and predominately in the open.
Valhalla is an example of the flag being partially open to sniper, but still on top of the base in the open. Several forged Halo 3 maps also demonstrated this principle.
Having the flag stand in the open not only allowed the defender to co-play offense in taking the enemy flag, it also allows for exciting, inch by inch movement of the flag to the goal when capturing the flag. As a team is taking the flag to their flag stand, their movement is at times in the open, especially as they reach those last few meters to the stand. With these pockets of exposure, the defending team finds high skill opportunities to stop the flag carrier in his tracks. Nail biting game play as the clock ticks down have been the case where the new flag carrier is summarily dropped by the sniper across the map. This feel of crossing the finish line with the flag is unique to Capture the Flag, Stock Pile, and Invasion.
As an aside, bomb games (Assault, One Bomb) require the bomb pad to be indoors so that the team arming the bomb has some protection. Yet, the bomb spawn should also be in the open in many cases to increase the risk of "going for the bomb". A good sniper on the enemy team can control the bomb at its spawn and give his team time to engage the enemy at their base where the bomb is spawning. This is a very competitive angle on bomb games.
When designing Capture the Flag maps, especially for asymmetrical CTF (One Flag), you should consider how the defenders will want to push out toward the middle of the map to engage the enemy sooner. In so doing, you should consider forward positions, such as sniper positions, that challenge the offense. At the same time the offense will need choices to counter these positions and strategies. The offense should be able to achieve control of the main contestable area of the map while the defenders should be able to "get into position" to challenge their control.
You don't want these defender positions being too powerful. A Rock, Scissors, Paper design where each power position has vulnerabilities that the other team can exploit keeps the players engaging in challenging ways.
And finally, a concept of "base complexity" should enter into your design. A base must have several ingress paths, but how those paths are structured can be critical to the fun of trying to invade the base.
One of the best examples of a base design with regard to complexity would be Standown. Each path into the base is extensive, unique, and requires different strategies to defend. Some allowed for stealth approach by crouching off radar and camo. Other examples include Last Resort, High Ground, and Long Bow. And example of a base that was extremely simplistic (not so good) would be Hekau, where you could walk up to the doorway fully exposed and see through the other doorway just steps away.
Making unique ingress paths into the base requiring different strategies keeps a defender interested in your map more than if the defense strategy is simple and across the board.
Exposing the flag to cross map fire allows defenders to also engage in offense play.
Maps forged for CTF need to include "crossing the finish line" game play experience.
Forward defense positions can help defenders stay near by their base while engaging the offense in interesting ways.
The base itself should exhibit complexities that make ingress styles unique and defense strategies varying.