I love this. But make it parallel to the bridge. You can walk across the bridge and never encounter the portal. But there's a spot in the middle of the bridge where the guide cables on one side attach to the support cable forming a 5 foot span with no guide cable on that side. Stepping off the bridge through that gap sends you through the portal.
Or something steps onto the bridge from the other side of the portal....
Pretty cool. I was thinking there was a teleportation circle in the middle, invisible to the untrained eye, that sends you someplace nearby - in a "killing zone" where someone collects a toll for crossing. It's close enough so they can keep an eye on the bridge. There's another portal that sends you back on the other side so you can continue on your perilous journey.
Maybe this lair that's been taken up by bandits/highwaymen was once the lair of a dragon/magical flying creatures (who created the portal), so the architecture of the place is not meant for humans.
The portal was inactive for a long time until the leader of the bandits - a wizard - found it and turned it on. Since then he's been sitting pretty but one of his lieutenants is getting restless; their "leader" never does anything but count gold coins. (To tie it together, maybe the wizard is a cultist of Tiamat.)
Here is what my random bandit generation chart came up with:
Level 5 bandit lair
28 human bandits
7 human guards
7 human berserkers
28 human crossbowmen
3 human nobles
2 half-elf bandit captains
1 human cavalier + 2 riding horses
1 human slaver
1 human knife fighter (Lieutenant) + 6 human guards (bodyguards)
1 gnome entropist (Leader, cultist of Tiamat)