Infantry are funneled because they are being threatened by other infantry. They can either stop and destroy the destructable, or engage the enemy threatening them. Here's an example of how seige shields and stakes can be used (yes, siege shields can funnel infantry too, especially at a doorway.)
TOP VIEW
O = Opposing team member
F = Friendly Team Member
X = seige shield or especially stakes
O O
X OX
X F X
F
Using such an arrangement, the Friendlies, who are outnumbered, can avoid being flanked. Sure, the enemies can stop and kill the destructables, but MOST OF THE TIME they do not and try to get a kill instead. Even if they do stop and attack the stakes/shields, they are wasting precious seconds and losing their number advantage. Same thing if they go around.
Yes, siege shields can be used to deny cav an area. They can be set up at a choke point, OR just put up to guard the flank of an archer.
Here is an example of how siege shields can be used to funnel enemies at door ways.
O = Opposing team member
F = Friendly Team Member
X = seige shield or stakes
_ = Walls
/ = Door
O O
______O _______
/ X
F FX
F
The door is a natural choke point, but using siege shields can extend the choke point and funnel players. The enemy players, if they want to go through the door, have no choice but to A) stop and attack the seige shield (and get struck), or B) enter and go to the left (their right) where more friendly players wait. Not a lot of players do this, but I do it, especially on siege maps. It can be really effective. Sometimes it doesn't work, but when it does its awesome!
