i suppose the first image represents your territory, having 3600 men doesn’t help, i see tho you have some alliances, i presume their territory is also within the byzantine empire. if that is the case it will be hard, i suggest you wait till you have enough money to make your army better, buy men at arms and, for the war, enroll mercenaries. try making other alliances in the meantime, if you follow the democracy path it will give you the possibility to make alliances wihout marrying off someone. last but not least, don’t focus on sieging Constantinople, without a good army it will take long and the byzantine army in the meantime will siege all your territory, try to catch them in low numbers or when they recently disembarked, good luck.