1.16.4, fabric loader 0.10.6
Mods:
battletowers-3.2.0-beta-1.16.4
fabric-api-0.25.4+1.16
lithium-fabric-mc1.16.3-0.5.6
modmenu-1.14.6+build.31
modupdater-1.1.11+1.16.2-rc2
phosphor-fabric-mc1.16.1-0.6.0+build.7
sodium-fabric-mc1.16.3-0.1.0

As mentioned by the Curseforge page description,
you need to get keys on each floor of the tower to unlock the boss at the top. There is no naturally spawning entity at the top of towers.