Bacon Bits
Legend
Imagine casting this on the only bridge across a major river as an army of orcs marches towards a helpless city. Or in a tunnel. Or the footbridge/drawbridge leading into a keep/castle. Or a stone paved road through a marsh. Or a mountainside path. Or major trade route between cities.
Assuming you have the full minute to cast it . . . wow. And it is a ritual, so you can just re-cast it every 8 hours, and with a good supply of rations/water or a create food/water spell you can just camp that spot until a caster with dispel magic or something similar removes it.
As a ritual it will take 10 minutes, but that's beside the point.
If it's an army, then an army has resources. It's not a PC group with 20,000 members. It will have combat engineers, and more than enough backs to get a ridiculous amount of manual labor done very quickly. Yes, even orcs have combat engineers. Why do you think they bring goblins?
> the only bridge across a major river
Collapse the bridge out of spite from their end. Build ferries. Cross river. Otherwise, go around.
> Or in a tunnel
Tunnel around. Then collapse the old tunnel from both sides. Otherwise, go around.
> footbridge
Cut the ropes. Then have some goblins cross and tie a lead to the bridge, haul it back up or start a second bridge, and cross. Otherwise, go around.
> drawbridge leading into a keep/castle
Why didn't you just raise the drawbridge? Hell, we'll just build a short ramp or use ladders to get over your dome and we can go right in! If that doesn't work, we'll just have to attack the walls. I mean, who attacks a castle and expects the gate to be open?
Sure, if you impose stupid restrictions like, "Its a bottomless gorge and this is the only bridge and you can't ever cross it any other way even magically," then sure, it works. In reality, an army doesn't set out without the capability and knowledge required to circumvent basic natural barriers, and *no* army is so stupid as to not have a plan B if a bridge, mountain pass, or some other natural barrier is taken by the enemy. Yes, it will take time. Days or weeks, possibly. However, all you've done is said, "What if I take this immense natural barrier that has a passage through or across it, and then removed or held the passage." That's been a tactic of war for thousands of years, from the Battle of Thermopylae to the German destruction of the bridges across the Rhine in WWII.