You didn't mention what race you play or he plays when you select Jotunheim, but I'm going to guess Norse based on the theme of the board.
This is a choke point board and controlling the pass certainly affects the outcome, however, you also need to play the strengths of your race.
For the Norse, their strengths are quick, powerful rushes early in the game. If you're spending time building up reserves or defences you aren't playing into their strength. If you can develop a quick offence, you can prevent the walls from going up and start removing his economic base, crippling him.
Maximizing your returns involves using your god powers effectively as well, because you mention your opponent making a defensive stand, a rush seems the best response.
Hersir Rush using Loki
Loki specialises in his heroes, the Hersirs. A typical Loki strategy is called the Hersir Rush. You use a fair number of Hersirs and the myth units they summon to overwhelm your opponent's early defences. Once the Hersirs start spawning myth units it often turns the tide.
To make this strategy work you must aggressively collect food. If your main food sources are berry bushes, then immediately start harvesting them. Train and add at least 4 more Gatherers to the mix. Once this is going you can explore the immediate vicinity of your base, looking for additional food sources.
If your main initial food source was hunted animals you should invest in the Hunting Dogs ability and start gathering from the wild life in the area. Try to avoid harvesting your cows or sheep until they've fattened. As you're able, add the 4 Gatherers to the hunting party.
Once your food production is moving along nicely, build a house near your base and also add an Ox Cart and 2 Gatherers to harvest wood from a nearby grove (don't chase down single trees yet). As you're able add an Ox Cart and 2 more Gatherers to harvest gold from the closest gold mine.
At this point your berry bushes should be nearly or totally depleted and you should move on to animal food sources if you haven't already. Adding 2 more food gatherers at this point will help keep things moving.
This is a good time to get your Ulfsark to build a Temple. You can keep it close to your base if you're being at all pressured, or you can set it towards the pass if your opponent is playing defensively.
Now add in 2 more gatherers on wood and 2-3 dwarves on gold.
When you're able, upgrade to the next age.
From this point on you're looking to generate your Hersir army. They are vulnerable to infantry but the myth units they spawn aren't, so you want to have enough Hersirs to withstand an initial counter attack.
If he has already built the wall you might want some raiders or other units to back you up, but if he hasn't built the wall then you should be able to do a lot of damage to him quickly.