@RC21 Flanking as in merely posing a threat of going around your enemy. You know, before shock cavalry introduction, the main strategic advantage of using one. Underground reinforcement is not that great of an Idea I think because there is not enough stopping power to prevent enemy from... Well, doing anything really. In the first place, ambushes aside (and it's not exactly ambush if enemy knows there are your fortifications) battle preparations could take hours in open view of each other, so you have all time in the world to get your forces where you want them to be. And as for ambushes, you have batter chances hiding some cavalry in hills and forests around rather then restricting yourself to single fighting area.
Regarding weaponary and magic level. Another thing that ircs me is that in many isekai stories locals are stupid. They made so because author needs to make main character (or just Japan) look smart
and author can't write character smarter them himself. As opposed to many misconceptions, people throughout the ages are not stupid and constantly making improvements in their trade. So people of that world should alredy be using the most of the resources and knowledge they have access to ensure their survival, so some brat from the future should not be able to improve that much in a field that lifes depend on. Warfare evolved rapidly answering the call of every new threat, getting something from the future would be as if putting anti-tank guns before actual introduction of mechanised forces (hello, Youjo Senki anime).
Introducing shock cavalty agains roman legions would pose a problem of lack of quality metal production, husbandry and sustainability. Introducing modern quick responce teams into WW2 would pose a problem of communication, ground coverage and strategic value. But by their own rights they fit in their respective era as perfect as possible.
Some isekaijin trying to teach locals how to build fortifications is aking to saying if you poke your enemy with a spear he might die. Genious?
Tunneling? Isn't it like it sounds? It is not that popular where I live, so probably have lots to do with climate zone and landscape. If you have natural caves and you don't live in a swapm - lucky you?