There are quite a few ways to do this but ideally first fix the limitations that this behavior tries to compensate for.. +8 is generally elevation bonus to range, aka the height of the turret itself. If you look into the selection tooltip it will show you total elevation bonus, which include how high on terrain the tower is (on a hill for example), however I found out recently that this wasn't correctly applied (see proposed fix for it). With arrow shooter tech it become 60base +8tech +9elevation total bonus (IIRC vanilla show them merged so +17). Stone towers also have higher base elevation bonus..

Here's another idea. Capture attack -> Siege attack. Buildings can be captured only if they have less than 50% of total HP (some techs could bring this percent down further). Default action for units is now siege attack. Capture is possible as before, but only after a structure is below 50% of HP. Otherwise, the capture attack is disabled (has red X across the capture icon, to indicate it's disabled). Siege attack animation is the same as the capture animation, but units are holding the torches up instead of their weapons.

Indeed, collisions solve that and other things. Cheaply massing troops becomes harder, as it should be, because the entry points to a fortification are limited. Still, capture points increasing with garrison could help. Another question to understand everything better: when saying tower range 10 to 60 (+8), what does the +8 mean? A change because of garrisons, or possible techs?

They should, if you have other units that are to engage the enemy. But, the issue here is that currently there is no soft cap on amount of units that can capture one building. That soft cap should be the size of the building, and when the enemy surrounds it completely, that's it. How many units can realistically surround the Fortress? Surely not 150.

Nice poll, @Atrik. Additionally, it would be good to address the exploit that allows capturing buildings in tight spaces using formations.