after having played as England and the Danes I gotta say, Russia is quite more challenging and well fun.

I'm around 1300 now and I didn't find much problem until now.

I think before, Poland was constantly trying to take Vilnius and Kiev, but were unsuccessful because I put full garrisons of spear and archers (cheap and effective).

I don't think using cavalry a lot of is a good idea since they're more expensive and taking cities with them is a pain in the butt.
Dismounted dvor however is a decent all around unit and I'm trying to build up a full army of them (they're highly armored warriors with very good missile skills, probably the best Russian infantry unit).

now, the way I started the game was by taking every visible rebel town, to the South and Southwest and then advanced east, taking the rest of the rebels. since I had 1 regions under my control, I had no problem with profits and the game was quite easy until now

What happened obviously the is the Mongolian invasion and it just so happens that as soon as they come, they lay a siege on Bulgar (eastern most town). but oddly enough, they do not take it and simply continue west. I assume it was a bug. they are advancing toward Kiev, so I'm massively evacuating the city since there's 4000 of them and I have less than an eight of that within 4 year of walk distance.

so I'm praying to my orthodox gods that they will only take kiev and continue west toward byzantine and hungary because there's not a chance right now that I can resist such an onslaught.
I also started making plans and decided I'll move to scandinavia is things go bad, so my fleet is standing by with some of the reserve forces.

also while fighting Poland, I managed to sack Iasi and another fortress near it that was lightly guarded and added about 10 000 total to my funds. it so happens that while they were sending all their forces to Riga, I moved south and cost them two cities (which Hungarians swiftly took).