"Furthermore, in 1987 OA [operational analysis] demonstrated that the defender is at a systematic disadvantage in close country (be it woods or built-up areas). It seems that, amongst other things, in close country the defender is generally unable to mass the fire of his weapons, due to very short ranges available in relation to unit frontages. Given their relative protection, if only from view, the attackers can mass forces more safely than is normal. They can therefore isolate and attack small bodies of enemy relatively easily. The overall effect was described as 'counterintuitive'. [...] In FIBUA [fighting in built-up areas; urban combat] the attacker is expected to suffer high casualties.
By assumption, the defender will suffer fewer casualties. Conversely it seems that such expectations, formed from experience of high casualties in FIBUA, are based on ignorance of relative casualty rates. Attacking infantry generally have an advantage of 3.57 :1 in terms of attackers' to defenders' casualties in FIBUA.
Jim Storr, The Human Face of War, p. 103
Now let's think this through. No matter whether it's accurate or not, it offers the opportunity for an interesting thought experiment.
The background is the growth of world population, the increasing urbanization, the sprawl of settlements over the landscapes (a trend which has halted at a saturation point in Western countries) and the fact that much if not most historical warfare was about especially densely populated regions. Resource-rich regions with few inhabitants rarely attracted much organized violence.
The dispersed settlements offer much concealment and some cover - they could be (mis?)understood as a modern replacement for the open terrain trench lines of the World Wars. An assumed superiority of the offence over the defence (at least inside settlements, not so much for attacks from open ground into settlements) suggests that such terrain is actually a poor choice for a stubborn defence. We lack the infantry quantity to pull it off anyway, at least without an impressive mobilization.
This would still suit the exploitation of settlements as concealment for small picket teams which exercise control of the surrounding open terrain through surveillance and engage hostiles primarily with indirect fire support.
Army troops might be forced to defend stubbornly in close terrain, though: A heavy (armour or mechanized infantry) brigade which lost too many of its armoured fighting vehicles (AFV) and lacks the strength to fight its way back to a friendly rear area might be forced to set up a large hedgehog defence in built-up or woods terrain.
Well, how should this be done? There are old (WW2) advices on how to eliminate a pocket. Besieging requires a comparable combat strength (a battalion can keep another battalion encircled), but the elimination requires a superiority of the attacker. This necessary superiority is often not at hand, and thus it becomes advisable to first split up the pockets into smaller pockets in order to be able to establish the required superiority at least over the smallest compartment.
The advisable general tactic is likely the employment of infantry with assault gun and mortar fire support.
At this point, the quote promotes a turn of tactics, away from the established ones:
The classic tactic would be to probe in order to find an opportunity, assign support and exploit the opportunity. Afterwards, a successful break-in would be reinforced and widened with reserves. It's striving for one initial success which has to be reinforced till mission accomplishment (manoeuvre à posteriori, later known as "recon pull").
The advantageous loss ratio from the quote is built on conditions which would not be met for long with such a tactic: The superior massing of attackers can and would be countered with a counter-concentration by the defender. The attack is bound to drop into a 1:1 fight with the intuitive tactical disadvantages of the attacker still in effect and now ruling the outcome.
A different tactic which would build on the findings mentioned by Mr. Storr could regain the advantage offered by unusually good concealment: A quick shifting of attacks from one point to another. OODA fans might love this. The quick concentration, attack, partial disengagement, new concentration, attack sequence could override the enemy's ability to counter-concentrate. The superiority gained by local numerical superiority, surprise and shock could indeed lead to very favourable exchange ratios. The numerical superiority and shock are both linked to the unusual reliability of suppression inside settlements. Buildings which were not reduced to rubble by bombardment offer relatively few potential firing positions. The quantity of windows, doors, flat roofs and removed roof tiles is rather limited in comparison to the hundreds of plausible fighting positions on open ground with drainage channels and vegetation.
In the end, urban combat could turn out to be very, very intense and offensive. Many small platoon attacks quickly shifting in emphasis from one spot to another could coin the battle.
The quick succession of many successful small-unit attacks could either lead to defeat of a pocket by attrition or it could serve to create a strength ratio which allows for the decisive, separating pushes or even an all-out concentric elimination attack.
A possible counter-tactic of the defender could be to revert to a picket line with strong and quick reserves, but this would be self-defeating for a pocket defender. Pocket defenders need to maintain as much terrain as possible, for pockets become ever more terrible the more your force is being compressed. A picket line would easily yield to an unfocused attack.
The findings about the attacker advantage in FIBUA mentioned in Storr's book are interesting, but don't appear to be very general if they're really about the superior ability to achieve tactical surprise. It's an interesting thought experiment to think about is implications and proper exploitation.