To defend a choke point in Ashes of the Singularity II, use blocking units to hold the narrow approach and put mortars or suitable ranged support behind them. The front slows the push while the support can fire on enemies that bunch up in the pass. Leave a route for both groups to withdraw if the blocker cannot hold.
Hold the part of the pass the enemy has to cross
Choose a position where your front line can cover the narrow route without spreading out. A blocking force only helps if it can remain between the attackers and your ranged units. Check the sides of the position for a route the enemy could use to bypass it.
The official Steam store page summarizes the October 28 developer video blog about two UEF forces fighting through a narrow pass. Its example pairs blocking units with mortar emplacements against closely clustered armored units.
Place the mortars behind the blocker
Keep the ranged support far enough back to avoid taking the first hit, while still giving it coverage over the part of the approach where attackers are likely to group. If the mortar position covers a spot the attacker can avoid, adjust the line or pick a different place to hold.
The published example concerns a UEF matchup. If you are using a different faction or set of units, apply the positioning idea to the ranged support you have and check its actual reach in the match.
Plan a fallback before the push begins
A narrow front can become a liability if the lead units collapse or the attacker finds another route. Pick a second position while you still have time to move. Keep the withdrawal route clear of reinforcements headed the other way and reposition the support before it is exposed.
Watch for two changes that justify moving: the blocking force is losing ground, or pressure shifts to a route that goes around it. Even a good mortar position loses its advantage if the enemy can reach it from the side.
FAQ
Why put blocking units in front of the mortars?
The blocking units contest the narrow route and help keep attackers away from your support. The mortars can then focus on enemies concentrated in the approach.
Does this work against every army composition?
The developer example features clustered armored units in a UEF fight. Check the enemy’s approach and how tightly it is grouped before committing to the same setup.
What if the attacker can go around the pass?
Cover the side route with a separate force if you can. If you cannot cover both routes, move to a position that gives your support a safer retreat.