Most of these are written for Ukrainians, so they contain little to no English. However, the pictures are quite clear, so I still feel that they are useful.
Use concrete as the primary defense material. An urban defender uses the unique urban material to their advantage. Concrete reinforced with steel bars (rebar) is ten times better than sand bags. Use rubble, rock, bricks.
Prepared urban defenders have the advantage. In 2016, it took over 100,000 security forces 9 months to take Mosul against 5 to 10,000 ISIS. It took hundreds of thousands of Soviets to take Stalingrad.
The underground has long served as a powerful urban warfare tool. It allows defenders to escape bombs, hide fighters, supplies and move around unseen, to include to attack the invading enemy.
The way the Ukrainians are fighting for their homeland have taken the Russians by surprise and it has cost them more resources and casualties than expected. If the untrained Ukrainians will fight like this, then it should give the Chinese and Russians pause if they should ever think about attacking the USA in spite of our feckless president. The American people at large are better armed and better trained than the Ukrainians could ever dream of being. Anyone who tries to attack America will be walking into a blood bath!