HTK
A few more modifications compared to prototype 7:
1) The outer bridges are now square, for more solidity
2) Stage 2 upward spiral walls are now more homogeneous, with 4 spiral walls along the circumference instead of 2
3) The trigger systems for the spiral wall are more reliable, with in-wall extra structural weight at the removable support wall start location
With this prototype, stage 1 and 2 are now very close to their final designs and true scale (except for the height) as all the requirements have been met:
destruction mechanisms reliability
size of colosseum diameter
overall stability during construction and stage 1 base width
levelness of the colosseum bridge base