I’ve about lost the will to live these last two days trying to finish river with randoms with a whole host of reasons but this morning I played with a party of 4 who didn’t speak english and booted me on wave 21 for not understanding their instructions, fair enough. But thankfully they were the first group I’ve encountered to not hold up in the town hall or cafe.
They put the fab in one of the positions I marked and it stops all spawning of enemies your side of the bridge, no need to hold up in the tower. They put fences on the bridge to protect the tap and some from underneath. After a while they pushed fences to the enemy side of the map and it became like a speed run. They could move to the tower if they wished but they killed everything before it got to the bridge.
I was watching the underpass but tbh not much came that way as they killed it from the bridge before it went under and leeches suicide under there by jumping at the characters on top of the bridge and smash against the underside.
Try it, put the fab there are taps are easier to hold and you can spread out a bit, far more fun! Hope that helps.