Is out sieging the AI an effective tactic?

[Agrajag]

No, you would crash the game.
I don't remember the limit, but I think its 32000 units, after that strange things begin to happen, and at 9999999999 I doubt the game will even work.
Also, if you try and move those units, you'll crash at hosting.
And besides, if you have 9999999999 city guards, someone else probably has 9999999999 of something better to take your city.

[Endoperez]

No. Your enemy could have 100 Mounted Commanders with Gate Cleavers and Wall Shakers, 10 of them would have Ring of Resist Lightning, and one A3 mage with Staff of Storms would enter with them and cast Wrathful Skies - they would also need some slow, high-hp or lightning-resistant cannon fodder to keep the commanders on the field, but it is doable.

[Chazar]

...and dont forget the 999something priests praying for your lone dominion candle in the wind...