Age of Strategy Forums

It is not a valuable asset to have in MP head-to-head with castle defensively (merely as defensive is what it can give) so lowering its current mend rate is considerable.

As monastery doesn't have any mend rate, I guess you mean mend affect or construction affect - so the speed at which it's repaired or constructed.
About construction - monastery is constructed quicker, than castle already (16 vs 18 turns for ambidexteria workers).
About repair - monastery is also quicker to repair, even if worker mends few hp less.

Yes, it is.

Continuing, is it still possible to make its construction goes quicker, 15 vs 18 of castle? Imho even with current construction speed, as mega building it doesn't promise much profit to invest in MP, even with its unique units.

Still allows making trebuchet, so I don't see a reason to make even quicker construction.
I see monastery as second line mega - good to provide support if player goes with line of units tactic.
But as it can be used for pure trebuchet construction factory it would be dangerous to make it too quick.

Imo what monastery needs is not a direct buff, but an upgrade, to make it more resistant and possibly add a conversion skill to it, similar to how high castles have trebuchet shots.

It's a monastery after all, non-sense for monastery to have shooting ability like castle. Aura or increasing hp via tech maybe viable, who knows.