In this tactic create two separate sub-cities, each separate from the other, with no roads, gardens, rubble of granaries to connect them. In one side, build a large group of Gladiator schools or lion houses. For reference, call this side, the supply side (A). On the other side, in the other sub-city, (call it the demand side(B)) build an Amphitheater or Coliseum. Have one road running from (A) outside your city, along the enemies anticipated approach. The road should follow a meandering zig zag path leading out from your (A) sub city exit, and eventually back toward your city, where it reenters at anther point toward (B). This meandering coiled road (Hence the name Cobra) is the only connection between sides A and B of your city, and because of this Gladiators or lions run along it's length from side A to get to side B, to enter the Coliseum of Amphitheater. The points at which A leaves the city, should be close to the point where B reenters, but the two points should be connected only by the long winding road.

The standing patrol of gladiators or lions along the road will block the largest enemy attacks as they zig zag back and forth in several successive undulations which intercept and bisect the enemy column of attack. These roads can be built to emanate from all four city sides, to block all possible attacks. You may cut the roads at a single place, at the three sides not under attack, and leave the roads connected for only the one side that is under attack.

I suggest you also build at least one Gladiator or Lion house, near the Coliseum or Amphitheater, so that structure is always working, and will immediately attract lions or gladiators from the other side of the city, when the road is connected.

A

B

Gladiators and lions will follow the road from A to B as long as it it the only connected road between A and B.
[Terratheon]

Comments
Two more variations on the Cobra defense
FLAMING COBRA
A variation of the cobra defenses, the flaming cobra uses javelin troops. Use javelins in bounding overwatch fighting retreat mode to lure the enemy into the cobra defense.
As the enemy fights his way though the successive parallell road sections filled with gladiators/Lions, position your javelin units behind the outermost road section the enemy has yet to break into.
The road sections filled with gladiators or lions are essentially successive parralell lines of defenders who bisect the enemy attack column at several points and each of these lines of defenders along each road is an obstacle behind which the javelin troops may withdraw for protection, as they continue to fire.
As the enemy brakes through each road section-line protecting the javelins, the peltasts may than simply withdraw behind the next road section and it's line of gladiators or lions. Very few attacks of any size, can penetrate even a few road sections. If they do penetrate the entire road network, your walls, ballista and legions can be waiting behind the array, and your javelins troops may withdraw behind their protection.

KNOTTED COBRA
The javelin units can be used in yet another way. As the enemy attacks your gladiators and lions along the undulating road, they will attempt to bypass the thickest groups of gladiators or lions. Use the javelin troops to snipe at the enemy troops, and than withdraw them behind these thicker groups.
The pursuing enemy will be lured toward the stronger groups of lions or gladiators with predictable results.
[Terratheon]