If you minimize route building (forcing the player to make a path to their exit) and sabotage (paths relatively close to each other, or crossing) then what are you left with?
All right, when I said 'route building and sabotage', what I really meant was sabotage and repair, i.e. the repetitive routine of digging and building in one spot. I think geoo's definition of 'route building' is the better one (constructing a route to get to your exit in the first place). I should edit my earlier post to clarify this.
I've noticed that on levels in which the start point is in the middle and exits on either end (so that paths don't cross) tend to make it very difficult to make a comeback because once a player has the lead they can strictly focus on path repair and effective sabotage becomes virtually impossible forcing a nuke instead of a certain slow death. So the tendency is to let weaker players lose by their own poor route building (losing a clone or two) and then just waiting for the map to end without ever making a move against the weaker player.
This comes down to map design. This problem can be avoided by delaying access to the exits, so that no lems are safe until the path is complete (see "Ghetto wars" and "Tower defense (Part 3)"), or requiring a dangerous path that can be very difficult to repair if sabotaged (see "Jenga"). In both cases, sabotage can be very effective until late on in the game. Besides that, geoo and Simon aren't weak players (far from it), and they need to entertain themselves
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At some point in every map you will have a completed path to your exit, and your next step (although better to do it in parallel) is to sabotage regardless of whether the paths cross. It seems like this is just a matter of degree and delay we are talking about.
I won't deny that. But, on maps where there is a delay before sabotage, players have a chance to gain an advantage (by speed and accuracy in route-building, or by better choice of routes) before the sabotage begins. On 'true' type 1 maps, there isn't such an opportunity, and the whole game is about sabotage. On top of that, there's the clustering factor, and all the tactical decisions associated with that.
Even with well designed levels that supposedly increase predictability of the location of sabotage i would think that crafty players would overcome that to sabotage in unexpected places.
On some maps (type 3, and early in type 2), typically your opponent can only reasonably get one or two lems into your base. You can only attack where you have lemmings, and you always can see where your opponent's lemmings are, so in these cases you can easily see where you could be attacked at any given time. When there are crossing streams, there are many possible points of attack, and many potential attackers, and then it becomes difficult to predict the point of attack (and nigh-on impossible if the path extends beyond the size of the screen).
As for the swap trap question, ccx provided a good response. It's the only trap that is specific to MP and is twice as deadly as a trap that just kills a clone so if you can lead your opponents clones into it there is a high reward. to truly alter gameplay you need to need to alter objective; what defines a win.
Ok, that's reasonable enough. But in adding more features and objectives, you risk losing the essential simplicity that makes the game fun to play. For the top players, this isn't so important, but a lot of people (especially new players, and their importance shouldn't be underestimated) just want to have fun, without being bogged down with rules.
I'll add that, regardless of what's been said so far about map types and their strategic value, what I like most when playing is variety. I've noticed that geoo and Simon like to stick to their dozen or so favourite maps, and immediately launch into a new game on the same map when they finish. My preference is to play as many different maps as possible, even mediocre ones, just to keep things fresh. Plus, as mentioned alrady, type 2 maps especially are quite demanding on the players, so it's nice to break from those after a while.